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0-30 VDC Stabilized power supply with current control 0.002-3 A
General Description
This is a high quality power supply with a continuously variable stabilised output adjustable at any value between 0 and 30VDC. The circuit also incorporates an electronic output current limiter that effectively controls the output current from a few milliamperes (2 mA) to the maximum output of three amperes that the circuit can deliver. This feature makes this power supply indispensable in the experimenters laboratory as it is possible to limit the current to the typical maximum that a circuit under test may require, and power it up then, without any fear that it may be damaged if something goes wrong.
There is also a visual indication that the current limiter is in operation so that you can see at a glance that your circuit is exceeding or not its preset limits.
This is a high quality power supply with a continuously variable stabilised output adjustable at any value between 0 and 30VDC. The circuit also incorporates an electronic output current limiter that effectively controls the output current from a few milliamperes (2 mA) to the maximum output of three amperes that the circuit can deliver. This feature makes this power supply indispensable in the experimenters laboratory as it is possible to limit the current to the typical maximum that a circuit under test may require, and power it up then, without any fear that it may be damaged if something goes wrong.
There is also a visual indication that the current limiter is in operation so that you can see at a glance that your circuit is exceeding or not its preset limits.
Technical Specifications - Characteristics
Input Voltage: ................ 24 VAC
Input Current: ................ 3 A (max)
Output Voltage: ............. 0-30 V adjustable
Output Current: ............. 2 mA-3 A adjustable
Output Voltage Ripple: . 0.01 % maximum
FEATURES
To start with, there is a step-down mains transformer with a secondary winding rated at 24 V/3 A, which is connected across the input points of the circuit at pins 1 & 2. (the quality of the supplies output will be directly proportional to the quality of the transformer). The AC voltage of the transformers secondary winding is rectified by the bridge formed by the four diodes D1-D4. The DC voltage taken across the output of the bridge is smoothed by the filter formed by the reservoir capacitor C1 and the resistor R1. The circuit incorporates some unique features which make it quite different from other power supplies of its class. Instead of using a variable feedback arrangement to control the output voltage, our circuit uses a constant gain amplifier to provide the reference voltage necessary for its stable operation. The reference voltage is generated at the output of U1. The circuit operates as follows: The diode D8 is a 5.6 V zener, which here operates at its zero temperature coefficient current. The voltage in the output of U1 gradually increases till the diode D8 is turned on. When this happens the circuit stabilises and the Zener reference voltage (5.6 V) appears across the resistor R5. The current which flows through the non inverting input of the op-amp is negligible, therefore the same current flows through R5 and R6, and as the two resistors have the same value the voltage across the two of them in series will be exactly twice the voltage across each one. Thus the voltage present at the output of the op-amp (pin 6 of U1) is 11.2 V, twice the zeners reference voltage. The integrated circuit U2 has a constant amplification factor of approximately 3 X, according to the formula A=(R11+R12)/R11, and raises the 11.2 V reference voltage to approximately 33 V. The trimmer RV1 and the resistor R10 are used for the adjustment of the output voltages limits so that it can be reduced to 0 V, despite any value tolerances of the other components in the circuit. Another very important feature of the circuit, is the possibility to preset the maximum output current which can be drawn from the p.s.u., effectively converting it from a constant voltage source to a constant current one. To make this possible the circuit detects the voltage drop across a resistor (R7) which is connected in series with the load. The IC responsible for this function of the circuit is U3. The inverting input of U3 is biased at 0 V via R21. At the same time the non inverting input of the same IC can be adjusted to any voltage by means of P2. Let us assume that for a given output of several volts, P2 is set so that the input of the IC is kept at 1 V. If the load is increased the output voltage will be kept constant by the voltage amplifier section of the circuit and the presence of R7 in series with the output will have a negligible effect because of its low value and because of its location outside the feedback loop of the voltage control circuit. While the load is kept constant and the output voltage is not changed the circuit is stable. If the load is increased so that the voltage drop across R7 is greater than 1 V, IC3 is forced into action and the circuit is shifted into the constant current mode. The output of U3 is coupled to the non inverting input of U2 by D9. U2 is responsible for the voltage control and as U3 is coupled to its input the latter can effectively override its function. What happens is that the voltage across R7 is monitored and is not allowed to increase above the preset value (1 V in our example) by reducing the output voltage of the circuit. This is in effect a means of maintaining the output current constant and is so accurate that it is possible to preset the current limit to as low as 2 mA. The capacitor C8 is there to increase the stability of the circuit. Q3 is used to drive the LED whenever the current limiter is activated in order to provide a visual indication of the limiters operation. In order to make it possible for U2 to control the output voltage down to 0 V, it is necessary to provide a negative supply rail and this is done by means of the circuit around C2 & C3. The same negative supply is also used for U3. As U1 is working under fixed conditions it can be run from the unregulated positive supply rail and the earth. The negative supply rail is produced by a simple voltage pump circuit which is stabilised by means of R3 and D7. In order to avoid uncontrolled situations at shut-down there is a protection circuit built around Q1. As soon as the negative supply rail collapses Q1 removes all drive to the output stage. This in effect brings the output voltage to zero as soon as the AC is removed protecting the circuit and the appliances connected to its output. During normal operation Q1 is kept off by means of R14 but when the negative supply rail collapses the transistor is turned on and brings the output of U2 low. The IC has internal protection and can not be damaged because of this effective short circuiting of its output. It is a great advantage in experimental work to be able to kill the output of a power supply without having to wait for the capacitors to discharge and there is also an added protection because the output of many stabilised power supplies tends to rise instantaneously at switch off with disastrous results.
First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier.
Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work.
In order to solder a component correctly you should do the following:
Mount the power transistor on the heatsink. To do this follow the diagram and remember to use the mica insulator between the transistor body and the heatsink and the special fibber washers to insulate the screws from the heatsink. Remember to place the soldering tag on one of the screws from the side of the transistor body, this is going to be used as the collector lead of the transistor. Use a little amount of Heat Transfer Compound between the transistor and the heatsink to ensure the maximum transfer of heat between them, and tighten the screws as far as they will go.
Attach a piece of insulated wire to each lead taking care to make very good joints as the current that flows in this part of the circuit is quite heavy, especially between the emitter and the collector of the transistor.
It is convenient to know where you are going to place every thing inside the case that is going to accommodate your power supply, in order to calculate the length of the wires to use between the PCB and the potentiometers, the power transistor and for the input and output connections to the circuit. (It does not really matter if the wires are longer but it makes a much neater project if the wires are trimmed at exactly the length necessary).
Connect the potentiometers, the LED and the power transistor and attach two pairs of leads for the input and output connections. Make sure that you follow the circuit diagram very care fully for these connections as there are 15 external connections to the circuit in total and if you make a mistake it may be very difficult to find it afterwards. It is a good idea to use cables of different colours in order to make trouble shooting easier.
The external connections are:
DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS UNDER POWER.
The voltmeter should measure a voltage between 0 and 30 VDC depending on the setting of P1, and should follow any changes of this setting to indicate that the variable voltage control is working properly. Turning P2 counter-clockwise should turn the LED on, indicating that the current limiter is in operation.
If you want the output of your supply to be adjustable between 0 and 30 V you should adjust RV1 to make sure that when P1 is at its minimum setting the output of the supply is exactly 0 V. As it is not possible to measure very small values with a conventional panel meter it is better to use a digital meter for this adjustment, and to set it at a very low scale to increase its sensitivity.
While using electrical parts, handle power supply and equipment with great care, following safety standards as described by international specs and regulations.
Voltages above 50 V are DANGEROUS and could even be LETHAL.
In order to avoid accidents that could be fatal to you or members of your family please observe the following rules:
Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit to see if there is a mistake there.
Parts List.
R1 = 2,2 KOhm 1W
R2 = 82 Ohm 1/4W
R3 = 220 Ohm 1/4W
R4 = 4,7 KOhm 1/4W
R5, R6, R13, R20, R21 = 10 KOhm 1/4W
R7 = 0,47 Ohm 5W
R8, R11 = 27 KOhm 1/4W
R9, R19 = 2,2 KOhm 1/4W
R10 = 270 KOhm 1/4W
R12, R18 = 56KOhm 1/4W
R14 = 1,5 KOhm 1/4W
R15, R16 = 1 KOhm 1/4W
R17 = 33 Ohm 1/4W
R22 = 3,9 KOhm 1/4W
RV1 = 100K trimmer
P1, P2 = 10KOhm linear pontesiometer
C1 = 3300 uF/50V electrolytic
C2, C3 = 47uF/50V electrolytic
C4 = 100nF polyester
C5 = 200nF polyester
C6 = 100pF ceramic
C7 = 10uF/50V electrolytic
C8 = 330pF ceramic
C9 = 100pF ceramic
D1, D2, D3, D4 = 1N5402,3,4 diode 2A - RAX GI837U
D5, D6 = 1N4148
D7, D8 = 5,6V Zener
D9, D10 = 1N4148
D11 = 1N4001 diode 1A
Q1 = BC548, NPN transistor or BC547
Q2 = 2N2219 NPN transistor
Q3 = BC557, PNP transistor or BC327
Q4 = 2N3055 NPN power transistor
U1, U2, U3 = TL081, operational amplifier
D12 = LED diode
Input Voltage: ................ 24 VAC
Input Current: ................ 3 A (max)
Output Voltage: ............. 0-30 V adjustable
Output Current: ............. 2 mA-3 A adjustable
Output Voltage Ripple: . 0.01 % maximum
FEATURES
- Reduced dimensions, easy construction, simple operation.
- Output voltage easily adjustable.
- Output current limiting with visual indication.
- Complete protection of the supplied device against over loads and malfunction.
How it Works
To start with, there is a step-down mains transformer with a secondary winding rated at 24 V/3 A, which is connected across the input points of the circuit at pins 1 & 2. (the quality of the supplies output will be directly proportional to the quality of the transformer). The AC voltage of the transformers secondary winding is rectified by the bridge formed by the four diodes D1-D4. The DC voltage taken across the output of the bridge is smoothed by the filter formed by the reservoir capacitor C1 and the resistor R1. The circuit incorporates some unique features which make it quite different from other power supplies of its class. Instead of using a variable feedback arrangement to control the output voltage, our circuit uses a constant gain amplifier to provide the reference voltage necessary for its stable operation. The reference voltage is generated at the output of U1. The circuit operates as follows: The diode D8 is a 5.6 V zener, which here operates at its zero temperature coefficient current. The voltage in the output of U1 gradually increases till the diode D8 is turned on. When this happens the circuit stabilises and the Zener reference voltage (5.6 V) appears across the resistor R5. The current which flows through the non inverting input of the op-amp is negligible, therefore the same current flows through R5 and R6, and as the two resistors have the same value the voltage across the two of them in series will be exactly twice the voltage across each one. Thus the voltage present at the output of the op-amp (pin 6 of U1) is 11.2 V, twice the zeners reference voltage. The integrated circuit U2 has a constant amplification factor of approximately 3 X, according to the formula A=(R11+R12)/R11, and raises the 11.2 V reference voltage to approximately 33 V. The trimmer RV1 and the resistor R10 are used for the adjustment of the output voltages limits so that it can be reduced to 0 V, despite any value tolerances of the other components in the circuit. Another very important feature of the circuit, is the possibility to preset the maximum output current which can be drawn from the p.s.u., effectively converting it from a constant voltage source to a constant current one. To make this possible the circuit detects the voltage drop across a resistor (R7) which is connected in series with the load. The IC responsible for this function of the circuit is U3. The inverting input of U3 is biased at 0 V via R21. At the same time the non inverting input of the same IC can be adjusted to any voltage by means of P2. Let us assume that for a given output of several volts, P2 is set so that the input of the IC is kept at 1 V. If the load is increased the output voltage will be kept constant by the voltage amplifier section of the circuit and the presence of R7 in series with the output will have a negligible effect because of its low value and because of its location outside the feedback loop of the voltage control circuit. While the load is kept constant and the output voltage is not changed the circuit is stable. If the load is increased so that the voltage drop across R7 is greater than 1 V, IC3 is forced into action and the circuit is shifted into the constant current mode. The output of U3 is coupled to the non inverting input of U2 by D9. U2 is responsible for the voltage control and as U3 is coupled to its input the latter can effectively override its function. What happens is that the voltage across R7 is monitored and is not allowed to increase above the preset value (1 V in our example) by reducing the output voltage of the circuit. This is in effect a means of maintaining the output current constant and is so accurate that it is possible to preset the current limit to as low as 2 mA. The capacitor C8 is there to increase the stability of the circuit. Q3 is used to drive the LED whenever the current limiter is activated in order to provide a visual indication of the limiters operation. In order to make it possible for U2 to control the output voltage down to 0 V, it is necessary to provide a negative supply rail and this is done by means of the circuit around C2 & C3. The same negative supply is also used for U3. As U1 is working under fixed conditions it can be run from the unregulated positive supply rail and the earth. The negative supply rail is produced by a simple voltage pump circuit which is stabilised by means of R3 and D7. In order to avoid uncontrolled situations at shut-down there is a protection circuit built around Q1. As soon as the negative supply rail collapses Q1 removes all drive to the output stage. This in effect brings the output voltage to zero as soon as the AC is removed protecting the circuit and the appliances connected to its output. During normal operation Q1 is kept off by means of R14 but when the negative supply rail collapses the transistor is turned on and brings the output of U2 low. The IC has internal protection and can not be damaged because of this effective short circuiting of its output. It is a great advantage in experimental work to be able to kill the output of a power supply without having to wait for the capacitors to discharge and there is also an added protection because the output of many stabilised power supplies tends to rise instantaneously at switch off with disastrous results.
Construction
First of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier.
Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.
DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work.
In order to solder a component correctly you should do the following:
- Clean the component leads with a small piece of emery paper.
- Bend them at the correct distance from the components body and insert he component in its place on the board.
- You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.
- Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board.
- When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder.
- The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it. Take care not to overheat the tracks as it is very easy to lift them from the board and break them.
- When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component.
- Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together.
- When you finish your work, cut off the excess of the component leads and clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain on it.
 connections.gif (17,8KB) |  pcb.gif (60KB) (12,5cm x 8,7cm) |
 layout.gif (92KB) |
Mount the power transistor on the heatsink. To do this follow the diagram and remember to use the mica insulator between the transistor body and the heatsink and the special fibber washers to insulate the screws from the heatsink. Remember to place the soldering tag on one of the screws from the side of the transistor body, this is going to be used as the collector lead of the transistor. Use a little amount of Heat Transfer Compound between the transistor and the heatsink to ensure the maximum transfer of heat between them, and tighten the screws as far as they will go.
Attach a piece of insulated wire to each lead taking care to make very good joints as the current that flows in this part of the circuit is quite heavy, especially between the emitter and the collector of the transistor.
It is convenient to know where you are going to place every thing inside the case that is going to accommodate your power supply, in order to calculate the length of the wires to use between the PCB and the potentiometers, the power transistor and for the input and output connections to the circuit. (It does not really matter if the wires are longer but it makes a much neater project if the wires are trimmed at exactly the length necessary).
Connect the potentiometers, the LED and the power transistor and attach two pairs of leads for the input and output connections. Make sure that you follow the circuit diagram very care fully for these connections as there are 15 external connections to the circuit in total and if you make a mistake it may be very difficult to find it afterwards. It is a good idea to use cables of different colours in order to make trouble shooting easier.
The external connections are:
- 1 & 2 AC input, the secondary of the transformer.
- 3 (+) & 4 (-) DC output.
- 5, 10 & 12 to P1.
- 6, 11 & 13 to P2.
- 7 (E), 8 (B), 9 (E) to the power transistor Q4.
- The LED should also be placed on the front panel of the case where it is always visible but the pins where it is connected at are not numbered.
DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS UNDER POWER.
The voltmeter should measure a voltage between 0 and 30 VDC depending on the setting of P1, and should follow any changes of this setting to indicate that the variable voltage control is working properly. Turning P2 counter-clockwise should turn the LED on, indicating that the current limiter is in operation.
Adjustments
If you want the output of your supply to be adjustable between 0 and 30 V you should adjust RV1 to make sure that when P1 is at its minimum setting the output of the supply is exactly 0 V. As it is not possible to measure very small values with a conventional panel meter it is better to use a digital meter for this adjustment, and to set it at a very low scale to increase its sensitivity.
Warning
While using electrical parts, handle power supply and equipment with great care, following safety standards as described by international specs and regulations.
CAUTION
This circuit works off the mains and there are 220 VAC present in some of its parts.Voltages above 50 V are DANGEROUS and could even be LETHAL.
In order to avoid accidents that could be fatal to you or members of your family please observe the following rules:
- DO NOT work if you are tired or in a hurry, double check every thing before connecting your circuit to the mains and be ready
- to disconnect it if something looks wrong.
- DO NOT touch any part of the circuit when it is under power.
- DO NOT leave mains leads exposed. All mains leads should be well insulated.
- DO NOT change the fuses with others of higher rating or replace them with wire or aluminium foil.
- DO NOT work with wet hands.
- If you are wearing a chain, necklace or anything that may be hanging and touch an exposed part of the circuit BE
CAREFUL.
- ALWAYS use a proper mains lead with the correct plug and earth your circuit properly.
- If the case of your project is made of metal make sure that it is properly earthen.
- If it is possible use a mains transformer with a 1:1 ratio to isolate your circuit from the mains.
- When you are testing a circuit that works off the mains wear shoes with rubber soles, stand on dry non conductive floor
- and keep one hand in your pocket or behind your back.
- If you take all the above precautions you are reducing the
- risks you are taking to a minimum and this way you are protecting
- yourself and those around you.
- A carefully built and well insulated device does not constitute any danger for its user.
- BEWARE: ELECTRICITY CAN KILL IF YOU ARE NOT CAREFUL.
Check your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.
Check again all the external connections to and from the circuit to see if there is a mistake there.
- See that there are no components missing or inserted in the wrong places.
- Make sure that all the polarised components have been soldered the right way round. - Make sure the supply has the correct voltage and is connected the right way round to your circuit.
- Check your project for faulty or damaged components.
Electronic Diagram.
Parts List.
R1 = 2,2 KOhm 1W
R2 = 82 Ohm 1/4W
R3 = 220 Ohm 1/4W
R4 = 4,7 KOhm 1/4W
R5, R6, R13, R20, R21 = 10 KOhm 1/4W
R7 = 0,47 Ohm 5W
R8, R11 = 27 KOhm 1/4W
R9, R19 = 2,2 KOhm 1/4W
R10 = 270 KOhm 1/4W
R12, R18 = 56KOhm 1/4W
R14 = 1,5 KOhm 1/4W
R15, R16 = 1 KOhm 1/4W
R17 = 33 Ohm 1/4W
R22 = 3,9 KOhm 1/4W
RV1 = 100K trimmer
P1, P2 = 10KOhm linear pontesiometer
C1 = 3300 uF/50V electrolytic
C2, C3 = 47uF/50V electrolytic
C4 = 100nF polyester
C5 = 200nF polyester
C6 = 100pF ceramic
C7 = 10uF/50V electrolytic
C8 = 330pF ceramic
C9 = 100pF ceramic
D1, D2, D3, D4 = 1N5402,3,4 diode 2A - RAX GI837U
D5, D6 = 1N4148
D7, D8 = 5,6V Zener
D9, D10 = 1N4148
D11 = 1N4001 diode 1A
Q1 = BC548, NPN transistor or BC547
Q2 = 2N2219 NPN transistor
Q3 = BC557, PNP transistor or BC327
Q4 = 2N3055 NPN power transistor
U1, U2, U3 = TL081, operational amplifier
D12 = LED diode
Scrolling LED sign based on Atmel ATtiny2313 AVR microcontrolle
On this page you will find a scrolling LED sign based on the ATtiny2313 AVR microcontroller, which you can build yourself (when finished) Other names for this device can be: Moving message sign, Message crawler, Scrolling message, message display, etc.
The idea is to let a text scroll over the LED dot-matrix displays. A dot-matrix display is a display which contains 5x7 dots (LEDs) in one case, the LEDs are connected like a matrix, there are two types CC and CA, the LEDs are simply put the other way around, here the drawings (inside and front):
The scanning goes as follows, first set the rows data on the 7 rows e.g. 1010010, then activate (0 or 1 -> depends on which type CA = common cathode, or CC = common anode) the first column, now these LEDs (dots) will lit, wait 5 msec, then switch the column off, now load the next rows data, and set the second column on, wait 5 msecs again, and switch it off again, if you repeat this sequence very fast, you will see the data (character data) appear on the display (refresh frequency 40 - 70Hz is ok, don't take twice or half the artificial light-frequency of 50/60 Hz)
The rows data comes e.g. from the EEPROM or flash memory of the AVR, you can also take an external EEPROM/flash IC, the ATtiny2313 has 128 bytes EEPROM and 2k of flash memory, what you can do is put the character data (ASCII) into the flash memory (read below for more details) Next the test-diagram:
The idea is to let a text scroll over the LED dot-matrix displays. A dot-matrix display is a display which contains 5x7 dots (LEDs) in one case, the LEDs are connected like a matrix, there are two types CC and CA, the LEDs are simply put the other way around, here the drawings (inside and front):
 |  |
The scanning goes as follows, first set the rows data on the 7 rows e.g. 1010010, then activate (0 or 1 -> depends on which type CA = common cathode, or CC = common anode) the first column, now these LEDs (dots) will lit, wait 5 msec, then switch the column off, now load the next rows data, and set the second column on, wait 5 msecs again, and switch it off again, if you repeat this sequence very fast, you will see the data (character data) appear on the display (refresh frequency 40 - 70Hz is ok, don't take twice or half the artificial light-frequency of 50/60 Hz)
The rows data comes e.g. from the EEPROM or flash memory of the AVR, you can also take an external EEPROM/flash IC, the ATtiny2313 has 128 bytes EEPROM and 2k of flash memory, what you can do is put the character data (ASCII) into the flash memory (read below for more details) Next the test-diagram:
The 74HC595 is an 8-bit shift-register IC, with this IC you can shift 8 bits to the outputs with only 3 wires, that are Data (Ds), and 2 shift inputs (SHcp, STcp), connect like the diagram. How does the 74HC595 works? First shift the 8 bits into the stages with SHcp, then shift the stages to the outputs with STcp, this causes the outputs to switch in one go, with e.g. a 74HCT164 you can only shift the bits into the outputs, the advantage of the 74HC595 is the storage register. Don't forget that multiplexing causes the LEDs only lit up for a fraction, so if you want the same intensity you must put more current through them, this diagram is for practice and programming, wants you have it working you can put transistors and resistors on. Here I put the letter R on the display as you can see, using a little breadboard: (next: How the scrolling is done...)
How to scroll a character accross the display ? The trick is to build one character on the display by scanning the columns very fast, and let say each 20 times (20 frames) scroll it one position to the left, this will give the effect of a walking text accross the dot-matrix display. So first build one frame, repeat this 20 times, and after that, read the data one address later, if you do this 5 times (5 columns) the character scroll from right to left from the display. (the refresh goes so fast that your brain can't keep up, and what you see is the R scrolling over the display) Btw, I will take five 74HC595's shiftregisters IC's, that are 5 x 8 bit = 40 bits / 5 columns = 8 dot-matrix displays, making it a nice tiny message sign.
What I am going to do is putting ASCII data (thats 128 x 5 bytes = 640 bytes) into the 2k flash memory of the ATtiny2313, then I have 704 words left for my program, that can really be a huge program!, because I used only 69 lines (69 instructions) of program so far, and that scrolls characters fluently accross the dot-matrix display. I made the program so that you can set the scroll-speed, from 0 - 255, so 256 speeds, 25 fps (frames/second) is a nice speed. On one of my pages (this page) I am using a 2-bit Gray code rotary encoder, with this encoder I will make an edit function in the software, so you can edit messages, without a keyboard, this save space, this type of rotary encoder has a push-function in the shaft, so e.g. after you select a character you can store that in memory.
A Digital, Up / Down Counter
Origionally I designed this for use on my Metal Laithe to Aid in winding coils.
But it can be adapted for many other applications.
This Circuit uses a CD40110BE, Up/Down Counter IC's.
The CD40110BE is Available from Digi-Key
Digi-Key Part Number "296-3506-5-ND"
But it can be adapted for many other applications.
This Circuit uses a CD40110BE, Up/Down Counter IC's.
The CD40110BE is Available from Digi-Key
Digi-Key Part Number "296-3506-5-ND"
**** DO NOT CONFUSE THIS IC WITH A CD4011 ****
This IC is able to Source Each Segment with 25 mA, Giving a Very Nice Bright Display.
The 7 Segment Displays MUST be a Common Cathode Type, as I have used here
NOTE Also : All the UnMarked Resistors should be at least 680 Ohms for Up To 12 Volts Supply Voltage.
For Higher Supply Voltages up to 18 volts or Reduced Currents, I would suggest Increasing these Values to 1500 Ohms.
Or if you want "Reduced Power" and "Brightness", Adjust the resistor values as appropriate.
Basically the Approximate Current is Supply voltage Minus 2, Divided by the Resistor Value.
The Schematic posted here Only shows the First, Second, Third and Last Stages.
And This board is for a counter of up to 9999.
However: Since All Stages between the Third and Last, Would be the same,
So you could make a display with as many digits as you wish, by expanding the circuit board.
Additionally: You can just put in 1, 2, 3, or all 4 IC's and the Appropriate Displays.
Other Options:
This IC is able to Source Each Segment with 25 mA, Giving a Very Nice Bright Display.
The 7 Segment Displays MUST be a Common Cathode Type, as I have used here
NOTE Also : All the UnMarked Resistors should be at least 680 Ohms for Up To 12 Volts Supply Voltage.
For Higher Supply Voltages up to 18 volts or Reduced Currents, I would suggest Increasing these Values to 1500 Ohms.
Or if you want "Reduced Power" and "Brightness", Adjust the resistor values as appropriate.
Basically the Approximate Current is Supply voltage Minus 2, Divided by the Resistor Value.
The Schematic posted here Only shows the First, Second, Third and Last Stages.
And This board is for a counter of up to 9999.
However: Since All Stages between the Third and Last, Would be the same,
So you could make a display with as many digits as you wish, by expanding the circuit board.
Additionally: You can just put in 1, 2, 3, or all 4 IC's and the Appropriate Displays.
Other Options:
- Adding a Clock Circuit with a Frequency of 1 Hz in place of the Reset Switch will create a Frequency counter in Hz/Sec.
- Adding a Clock Circuit with a Frequency of 1 Hz into one of the Inputs can create an up or down counter type of timer.
- Although Not Highly Accurate, a Simple 555 circuit will work as a Simple Clock.
Home security system
The security system application and program offers a simple demonstration of the BASIC Serial Interface. By adding only a few door and window switches, a transistor, a siren, (see schematic) and a few lines of BASIC program (see program listing) the interface can become a multi-function security system. Please note, however, that it is a "barebones" program. It is left to the reader to fancy it up to their liking.
Normally closed door and window switches can be attached to the interface "in" ports as shown in the schematic(all unused ports should be grounded). In this configuration with all the switches closed the "in" port is held "low". When any switch opens the port goes "high". The program recognizes this as an alarm condition for the zone associated with that port. If the program "detects" a high on "in" port number 1 it will delay sounding the alarm for a user defined length of time. This is done to allow the owner time to enter the secured area and reset the alarm before the siren is activated. If a "high" condition is detected on any of the other ports, 2 to 7, an alarm will be sounded immediately.
Normally closed door and window switches can be attached to the interface "in" ports as shown in the schematic(all unused ports should be grounded). In this configuration with all the switches closed the "in" port is held "low". When any switch opens the port goes "high". The program recognizes this as an alarm condition for the zone associated with that port. If the program "detects" a high on "in" port number 1 it will delay sounding the alarm for a user defined length of time. This is done to allow the owner time to enter the secured area and reset the alarm before the siren is activated. If a "high" condition is detected on any of the other ports, 2 to 7, an alarm will be sounded immediately.
The alarm is sounded by bringing "out" port number 1 high. Connected to "out" port 1 is a NPN transistor which switches a 12 volt supply to a security siren or bell (figure 3) . The.alarm remains on until the system is reset or it reaches it's time out period.
In order for the BSI to transmit the status of it's "in" ports Data Strobe (pin. 23 of IC1) must be toggled. This toggling of the Data Strobe is done by program control. In this application Data Strobe is connected to "out" port 8 by a jumper. In order to trigger a transmission of the port conditions the program turns "out" port8 "on" then "off". This causes IC1 to transmit the status of it's "in" ports.
10 ' BASIC SERIAL INTERFACE
20 '
30 ' SECURITY SYSTEM DMONSTRATION PROGRAM
40 '
50 ' setup
60 KEY OFF:CLS:CLOSE'......................................... turn key off, clear screen, close
70 OPEN "COM1:1200,N,8,2" AS #1' .......................all files, open the serial port
80 PRINT#1,CHR$(NUL);'.........................................as com port #1, and transmit "0".
90 GOTO 310
100 '
110 FOR X=1 TO 8'.................................................. Subroutine to convert decimal number
120 B=C MOD 2:C=INT(C/2):R(X)=B'...................... received from the UART to binary
130 NEXT X'............................................................. and set array variables to represent
140 RETURN'.............................................................UART port conditions,R(1) to R(8)
150 REM
160 IF T(HP)=1 THEN 210'.......................................Subroutine to turn one UART port on
170 FOR X=1 TO 8'..................................................without changing the condition of
180 IF HP=X THEN OT=OT+2^(X-1):T(X)=1'...............any other UART port.---
190 NEXT X
200 PRINT #1,CHR$(OT);
210 RETURN
220 '
230 IF T(HP)=0 THEN 280'........................................Subroutine to turn one UART port off
240 FOR X=1 TO 8'...................................................without changing the condition of
250 IF HP=X THEN OT=OT-2^(X-1):T(X)=0'................any other port.---
260 NEXT X
270 PRINT #1,CHR$(OT);
280 RETURN
290 '********************* SECURITY SYSTEM MAIN PROGRAM *******
300 '
310 PRINT" Security System Program
320 '
330 PRINT:PRINT"Note:'OUT' port 8 of the UART (pin 5) must be connected to Data Strobe (pin 23)before running this program.":PRINT
340 INPUT"ENTER ALARM DELAY FOR ZONE #1 ENTRY ";DELAY
350 INPUT"ENTER ALARM TIMEOUT ";TIMEOUT
360 '
370 CLS:PRINT#1,CHR$(128);'.................................clear screen and turn UART port 8 on
380 PRINT "Ctrl E to reset"
390 HP=8 :GOSUB 220:HP=8:GOSUB 150'................Ask UART for 'in' port status.
400 IF LOC(1)=0 THEN 470'.......................................If transmission not received,skip.
410 IN$=INPUT$(1,#1):C=ASC(IN$):GOSUB 100'... read transmission and convert to
420 FOR X=1 TO 8'......................................................binary,assign each bit to array R(X)
430 LOCATE X+9,10
440 IF R(X)=1 THEN PRINT X;" ALARM !!!!!"' Print UART port status conditions
450 IF R(X)=0 THEN PRINT X;" ZONE SECURE"' as either 'alarm' or 'secure'
460 NEXT X
470 IF R(1)=1 THEN TIME=TIME+'.............................If zone 1 is high start delay time.
480 IF TIME=DELAY THEN ALARM=1'.......................if delay time is up set alarm.
490 FOR X=2 TO 8
500 IF R(X)=1 THEN ALARM=1'..................................if any zone,2-8,is high,set alarm.
510 NEXT X
520 IF ALARM=1 THEN HP=1:GOSUB 150'............... if alarm set,turn port 1 on.
530 IF ALARM=1 THEN RESETT=RESETT+1'............ if alarm is set start timeout.
540 IF RESETT=TIMEOUT THEN GOTO 580'............ If timeout is up then shutdown.
550 A$=INKEY$:IF A$="" THEN 570'........................ Check to see if Ctrl E was entered,
560 IF ASC(A$)=5 THEN 50'...................................... if it was then reset program.
570 GOTO 390
580 PRINT#1,CHR$(NUL);'.......................................Turn alarm off
590 PRINT:PRINT"SYSTEM SHUTDOWN AT "TIME$,DATE$ ' print shutdown
600 END
In order for the BSI to transmit the status of it's "in" ports Data Strobe (pin. 23 of IC1) must be toggled. This toggling of the Data Strobe is done by program control. In this application Data Strobe is connected to "out" port 8 by a jumper. In order to trigger a transmission of the port conditions the program turns "out" port8 "on" then "off". This causes IC1 to transmit the status of it's "in" ports.
10 ' BASIC SERIAL INTERFACE
20 '
30 ' SECURITY SYSTEM DMONSTRATION PROGRAM
40 '
50 ' setup
60 KEY OFF:CLS:CLOSE'......................................... turn key off, clear screen, close
70 OPEN "COM1:1200,N,8,2" AS #1' .......................all files, open the serial port
80 PRINT#1,CHR$(NUL);'.........................................as com port #1, and transmit "0".
90 GOTO 310
100 '
110 FOR X=1 TO 8'.................................................. Subroutine to convert decimal number
120 B=C MOD 2:C=INT(C/2):R(X)=B'...................... received from the UART to binary
130 NEXT X'............................................................. and set array variables to represent
140 RETURN'.............................................................UART port conditions,R(1) to R(8)
150 REM
160 IF T(HP)=1 THEN 210'.......................................Subroutine to turn one UART port on
170 FOR X=1 TO 8'..................................................without changing the condition of
180 IF HP=X THEN OT=OT+2^(X-1):T(X)=1'...............any other UART port.---
190 NEXT X
200 PRINT #1,CHR$(OT);
210 RETURN
220 '
230 IF T(HP)=0 THEN 280'........................................Subroutine to turn one UART port off
240 FOR X=1 TO 8'...................................................without changing the condition of
250 IF HP=X THEN OT=OT-2^(X-1):T(X)=0'................any other port.---
260 NEXT X
270 PRINT #1,CHR$(OT);
280 RETURN
290 '********************* SECURITY SYSTEM MAIN PROGRAM *******
300 '
310 PRINT" Security System Program
320 '
330 PRINT:PRINT"Note:'OUT' port 8 of the UART (pin 5) must be connected to Data Strobe (pin 23)before running this program.":PRINT
340 INPUT"ENTER ALARM DELAY FOR ZONE #1 ENTRY ";DELAY
350 INPUT"ENTER ALARM TIMEOUT ";TIMEOUT
360 '
370 CLS:PRINT#1,CHR$(128);'.................................clear screen and turn UART port 8 on
380 PRINT "Ctrl E to reset"
390 HP=8 :GOSUB 220:HP=8:GOSUB 150'................Ask UART for 'in' port status.
400 IF LOC(1)=0 THEN 470'.......................................If transmission not received,skip.
410 IN$=INPUT$(1,#1):C=ASC(IN$):GOSUB 100'... read transmission and convert to
420 FOR X=1 TO 8'......................................................binary,assign each bit to array R(X)
430 LOCATE X+9,10
440 IF R(X)=1 THEN PRINT X;" ALARM !!!!!"' Print UART port status conditions
450 IF R(X)=0 THEN PRINT X;" ZONE SECURE"' as either 'alarm' or 'secure'
460 NEXT X
470 IF R(1)=1 THEN TIME=TIME+'.............................If zone 1 is high start delay time.
480 IF TIME=DELAY THEN ALARM=1'.......................if delay time is up set alarm.
490 FOR X=2 TO 8
500 IF R(X)=1 THEN ALARM=1'..................................if any zone,2-8,is high,set alarm.
510 NEXT X
520 IF ALARM=1 THEN HP=1:GOSUB 150'............... if alarm set,turn port 1 on.
530 IF ALARM=1 THEN RESETT=RESETT+1'............ if alarm is set start timeout.
540 IF RESETT=TIMEOUT THEN GOTO 580'............ If timeout is up then shutdown.
550 A$=INKEY$:IF A$="" THEN 570'........................ Check to see if Ctrl E was entered,
560 IF ASC(A$)=5 THEN 50'...................................... if it was then reset program.
570 GOTO 390
580 PRINT#1,CHR$(NUL);'.......................................Turn alarm off
590 PRINT:PRINT"SYSTEM SHUTDOWN AT "TIME$,DATE$ ' print shutdown
600 END
12 VOLT FLASHING BEACON
Note: The lamp shown is not supplied unless requested. A festoon bulb is supplied.
Note: Aligator clips may replace the auto plug.
Note: Aligator clips may replace the auto plug.
Schematic Diagram
PCB LayoutConstruction
2. A Circuit Board Holder is useful in freeing both hands for mounting and soldering the components. A spring board clip can be screwed down and will hold the board securely. Wooden blocks with a suitable groove, and able to slide (say on two pieces of dowel) to adjust to the varying lengths of PC Board also work well. Commercially produced stands are available, with an alternative magnifying glass. (types and prices available).
3. Mounting the components is relatively easy. The resistors will have to have the legs bent to match their holes. They may either be mounted to stand vertically or flush on the PCB, except, please note that R5 must be mounted flat on the board to clear under the body of the power transistor.
4. The power diode has a band at the neg (K) end. Polarity must be observed. (This diode is there to protect the unit from the effects of connecting it to the battery in the wrong polarity). The Electrolytic Capacitor also is polarised. There is an arrow on the body pointing down the negative (K) leg. You will notice that the Trimpot has one outside and centre legs joined in the track. This is correct; the other outside leg goes to the other track as indicated.
5. The small Transistor (BC558) has a flat on the body. The transistor is mounted with the flat towards the large transistor. The Power Transistor (2N3055) will fit the holes perfectly, so it is unlikely you will mount it the wrong way round. Note that one hole in the track is drilled D3mm to match the hole in the metal body. The metal body is the Collector (C) of this transistor. Use the 3mm machine screw and two nuts to join it to the track. Feed the screw down through the transistor hole and put a nut top and bottom of the PCBoard. The legs can then be soldered.
6. The six pins can be pushed into their holes. Soldering Technique is most important.
7. If you are fitting a switch to allow the unit to be have a trouble light mode then this switch is connected (by wires) to Pins 2 and 3.
8. Test the unit and adjust the flash rate with the trimpot. N.B.: This unit will work only on FULLY RECTIFIED CURRENT. It will NOT work from a power-pak plugged into a power outlet. A car battery is best but a 9Volt battery will work. If the unit doesn't work, check polarity of all components (diode,capacitor,transistors). Check the colour bands to ensure resistors are in correct places. Check that the trimpot bridges across the tracks with one outside and centre legs. Visually check soldering; resolder any suspect joints. Check for bridges of solder between tracks and remove them. The circuit is reliable and robust. Exercise meticulous care in putting it together and it will fire up first time.
If you are electing to construct the Flashing Beacon via the template/insulator method, the template appears below
INFRA-RED DOOR ALARM
Transmitter Construction
- Inspect the tracks for fine breaks and test the continuity of each track with an electronic circuit tester or ohm-meter.
- Identify the four resistors and insert them in the PCB according to the PCB Layout diagram.
- The 8 pin IC socket can be pushed carefully into place. Solder in place.
- There are two capacitors. Their values are marked on the body, they can go in any way round. Solder in place.
- The four PCB pins can be inserted and soldered.
- The IR LED can be identified by it's purple colour.Observe polarity and insert and solder,
- The second LED is inserted and soldered, observing polarity.
- The switch can be located and soldered.
- Find the locating DOT on the 555 IC and arrange it as the drawing shows. To fit the 555 it will probably be necessary to bend the legs inwards a little by placing one set of legs on a flat surface and push down lightly. This will maintain their alignment. Repeat for the other side of the chip so they match the socket. Push the IC into place.
- The battery snap can be soldered to the pins.
- The transmitter uses a 3V power source. The 2 x AA battery holder is used for the transmitter. Connect the batteries.
- When the switch is in the ON opsition, the LED will illuminate.
Trouble shooting
Troubleshooting if necessary will involve careful checking of locations and polarity of components, mainly the LED polarity and the locating DOT on the 555 IC is as the drawing shows. Re-solder all joints and check to make sure you have not bridged across between any two adjacent component legs. The end of a broken hacksaw blade sharpened on an emery wheel is a good tool for cleaning between soldered joints.Receiver Construction
- Inspect the tracks for fine breaks and test the continuity of each track with an electronic circuit tester or ohm-meter.
- Insert the capacitor and solder.
- Insert the transistor observing orientation, solder in place.
- Insert and solder the PCB pins.
- Locate the switch and solder.
- Connect the buzzer wires to the PCB pins.
- Insert the IR receiver and solder.
- Connect the battery holder to the PCB pins.
- Insert the batteries. When the receiver is on, the buzzer will sound.
Setting up
Arrange the transmitter and receiver about 1 metre apart with the IR LED of the transmitter pointing in the direction of the receiverOrientate the receiver so that the IR decoder is facing the transmitter.
Turn the receiver ON - the buzzer will sound.
Turn the transmitter on and the buzzer should stop. If not check the orientation of both transmitter and receiver.
If you place your hand (or some object) in the beam, the buzzer will sound.
This unit has a range of 2 - 3 metres.
Trouble shooting
Troubleshooting if necessary will involve careful checking of locations and polarity of components. Re-solder all joints and check to make sure you have not bridged across between any two adjacent component legs. The end of a broken hacksaw blade sharpened on an emery wheel is a good tool for cleaning between soldered joints.Technical notes
Looking at the transmitter, the 555 creates a 50 kHz signal which is used to power the IR LED. This sends a beam of infra-red light.The frequency of oscillation can be calculated using the following formula:
The receiver is primarily made up of the IR decoder. This component receives the IR transmission and converts it back into the 50 kHz signal that was generated by the transmitter - appearing at the Data pin. This signal is filtered by the capacitor which stores enough energy to maintain the status of the transistor in the off position until the next signal pulse (50 kHz - 50 000 times a second).
When the beam is broken, the transistor is energised and the buzzer sounds.
Touch activated alarm system
Parts List
R1 = 100K D1 = 1N4004, (or any other 1N4001,2,etc) general purpose diode
R2 = 4K7 C1 = 47uF/16V, electrolytic
R3 = 10M C2 = 0.1uF (100nF) ceramic
P1 = 100K IC1 = 555 Timer
Ry = Relay Q1 = 2N3904, 2N2222, or similar
Additional Notes
Not much to tell here as the circuit speaks for itself. The 555 can be almost any type, they are all pin-compatible. Although some CMOS types may not have enough power to drive the transistor, in that case use an ordinary 555. C1's working voltage should be increased to 25V if you decide to go with a 12V power source. Change the value of C1 for the desired output pulse.
For the timing use this equation: T=1.1*(R1+P1)*C1 assuming R1 + P1 = 150K, then select C1 as follows: C1 = 6uF for each 1-second pulse width. For example, if you want the pulse width to be 5 seconds, C1 should be 30uf or nearest value like 22 or 33uF. Additionally, P1 can adjust the rest.
Rule of thumb: the working voltage of capacitors are at least double the supplied voltage, in other words, if the power source is 9Volt, your capacitor(s) is at least 18V. Transistor T1 can be any approximate substitute. Use any suitable relay for your project and if you're not tight on space, use any size. I've build this particular circuit to prevent students from fiddling with the security cameras in computer labs at the University I am employed. I made sure the metal casing was not grounded. But as the schematic shows you can basically hook it up to any type of metal surface. I used a 12-vdc power source. Use any suitable relay to handle your requirements. A 'RESET' switch (Normally Closed) can be added between the positive and the 'arrow-with-the-+'. The trigger (touch) wire is connected to pin 2 of the 555 and will trigger the relay, using your body resistance, when touched. It is obvious that the 'touching' part has to be clean and makes good contact with the trigger wire. This particular circuit may not be suitable for all applications. Just in case you wonder why pin 5 is not listed in the schematic diagram; it is not really needed. In certain noisy conditions a small 0.01uF ceramic capacitor is placed between pin 5 and ground. It does no harm to add one or leave it out.
NOTE: For those of you who did not notice, there is an approximate 5-second delay build-in before activation of the relay to avoid false triggering, or a 'would-be' thief, etc.
AGAIN, make sure the latch (pin 2) is not touching anything 'ground' or the circuit just keeps resetting itself and so will not work. My shed has wooden doors so works fine. If you can't get yours to work, check the trigger input, verify there is some sort of signal coming from output pin 3, play with the value of R3/C1, etc.
The original circuit, as submitted by W. Knight to Hands-on magazine, was as shown below. R2 is replaced by the two resistors and the 33uF capacitor for the delay.
Not much to tell here as the circuit speaks for itself. The 555 can be almost any type, they are all pin-compatible. Although some CMOS types may not have enough power to drive the transistor, in that case use an ordinary 555. C1's working voltage should be increased to 25V if you decide to go with a 12V power source. Change the value of C1 for the desired output pulse.
For the timing use this equation: T=1.1*(R1+P1)*C1 assuming R1 + P1 = 150K, then select C1 as follows: C1 = 6uF for each 1-second pulse width. For example, if you want the pulse width to be 5 seconds, C1 should be 30uf or nearest value like 22 or 33uF. Additionally, P1 can adjust the rest.
Rule of thumb: the working voltage of capacitors are at least double the supplied voltage, in other words, if the power source is 9Volt, your capacitor(s) is at least 18V. Transistor T1 can be any approximate substitute. Use any suitable relay for your project and if you're not tight on space, use any size. I've build this particular circuit to prevent students from fiddling with the security cameras in computer labs at the University I am employed. I made sure the metal casing was not grounded. But as the schematic shows you can basically hook it up to any type of metal surface. I used a 12-vdc power source. Use any suitable relay to handle your requirements. A 'RESET' switch (Normally Closed) can be added between the positive and the 'arrow-with-the-+'. The trigger (touch) wire is connected to pin 2 of the 555 and will trigger the relay, using your body resistance, when touched. It is obvious that the 'touching' part has to be clean and makes good contact with the trigger wire. This particular circuit may not be suitable for all applications. Just in case you wonder why pin 5 is not listed in the schematic diagram; it is not really needed. In certain noisy conditions a small 0.01uF ceramic capacitor is placed between pin 5 and ground. It does no harm to add one or leave it out.
NOTE: For those of you who did not notice, there is an approximate 5-second delay build-in before activation of the relay to avoid false triggering, or a 'would-be' thief, etc.
AGAIN, make sure the latch (pin 2) is not touching anything 'ground' or the circuit just keeps resetting itself and so will not work. My shed has wooden doors so works fine. If you can't get yours to work, check the trigger input, verify there is some sort of signal coming from output pin 3, play with the value of R3/C1, etc.
The original circuit, as submitted by W. Knight to Hands-on magazine, was as shown below. R2 is replaced by the two resistors and the 33uF capacitor for the delay.
Motion Triggered Spy Cam
| step 1 | What You Need... |
 |  |
1. Motion Sensing Door Chime. Radio Shack Part #49-426
2. Mini Spy Cam DVR from Ebay
3. 200uF Capacitor
4. Any Reed Relay
5. Diode 1N4001, 1N4007, etc.
6. Momentary Switch
| step 2 | Prepare the Motion Sensor |
Disasemble the Motion Sensor. There is opnly one screw under the battery compartment. Remove it and then pry apart the case. Cut or desolder the speaker leads and the external output cable and connector. This should leave you with just the circuit board and the 9V power connector.
| step 3 | Assemble the Components |
Time to assemble the components that link the motion sensor and the DVR. Follow the included schematic. 1. Solder the diode to the reed relay paying attention to the cathode (-) position. The purpose of the diode is to prevent voltage flow back into the circuit when the relay is triggered. 2. Solder the capacitor to the diode. Again, pay attention to the striped (-) marking on the capacitor. The purpose of the capacitor is to turn the relay into a temporary switch by limiting the voltage pulses to a single pulse rather than the continual pulses the alarm gives out. 3. Make your connection from the (+) side of the cap to J2 on the circuit board. J2 is the trigger point for the motion sensor. 4. Make your connection from the anode (+) side of the diode to ground located where the 9V wires are at J5 on the circuit board. 5. Run two wires from the outside contacts on the relay to your switch. The switch allows you to stop recordin and power off the DVR.
| step 4 | Prepare the DVR |
This is tricky. On the end of the mini DVR is a super-micro push button switch that must be de-soldered and removed. This may require an assistant to carefully lift up on each side with a tiny screwdriver as you heat the contacts. Once the switch is removed, solder two leads to the two contacts. Then wire the two leads to the outside contact on the reed relay. Done!
| step 5 | Test it Out |
At this point you should be able to press the momentary switch and the DVR should power up (yellow indicator) and begin recording (blue indicator). Then attach a 9v battery and slide the switch on the motion sensor to the alarm position. (towards the 9v wires). There is a built-in 15 second delay when you turn it on to allow you to place the sensor and walk away. After that you should be able to wave your hand across the sensor and the DVR should power up and begin recording. While the sensor lens increases the range to around 30', eliminating it will reduce that distance so keep that in mind when you place it in service. The DVR is USB based. The movie files (352 X 288 12fps) are located in the "movies' folder when you plug in the DVR to your computer. Have Fun, Be Safe and use responsibly.
TOUCH Switch
All the circuits and projects we describe in these articles consist of very important "building blocks" that you can add to other designs. INPUT OUTPUT INPUT OUTPUT
This time we describe the concept of touching a plate (or two plates separated by a small gap) and turning a circuit ON or OFF.
A TOUCH PLATE is classified as a high impedance device (or high impedance circuit) as the effect of a finger will be detected by the circuit connected to the plate.
To learn more about the concept of high impedance circuits, see Page 77 of our Basic Electronics course.
If only a single plate is present, the circuit will actually be picking up "mains hum" from the finger. To prove this, take the project into an open space such as a large park and try the circuit. It will not work.
If the plate has a signal on it (from an oscillator), the effect of your finger will be to remove the signal (or reduce its amplitude considerably) and a detecting circuit will be activated.
If the circuit has two plates, it will be registering the resistance of your finger. If the circuit has 4 plates, it will use two to turn the circuit ON and two to turn the circuit OFF.
There are a number of different types of TOUCH PLATES and different effects can be created by the circuit.
1. Touch a set of pads and the project turns on. When the finger is removed, the circuit turns off. The finger can touch the pads for any length of time. We also include the feature where the circuit extends the ON period, so the circuit stays on for a length of time after the finger is removed. This is shown in Circuits A.
2. Touch a set of pads fairly quickly and the project turns on. Touch the pads again for a short period of time and the circuit turns off. This is called the "Flip-Flop" effect. If the finger is kept on the pads, the circuit will turn on-off-on-off at a rate of about once per second. This is shown in Circuits B.
3. Touch one set of pads to turn the circuit on and another set of pads to turn the circuit off.
This is shown in Circuits C.
CIRCUITS A
Here are a number of circuits that turn on a device when the touch-pad is touched.
The circuit above is the simplest Touch Switch. It is called a "super-Alpha pair" and is actually identical to a single transistor with a very high gain.
Putting a finger on the touch pads turns the top transistor ON and this transistor turns on the bottom transistor. When the finger is removed, the circuit consumes less than a microamp.
The 555 can be used to create a Touch Switch. The only problem with this is the 555 consumes about 8mA, at all times when the supply is connected. The circuit above turns on the LED when the finger is applied and pin t becomes "open circuit." This allows the 10u to charge via the 100k resistor and when pin 6 detects a HIGH, the LED turns off. The finger should be removed before this occurs. See below for an ON-OFF touch switch using a 555.
The Touch Switch circuit above is a very complex design to do a simple task. It is also a very poor design as the biasing (turn-on) for the output transistor is via a resistor and the output transistor is turned off by taking the biasing current to the 0v rail. This is a wasteful design if the circuit is to be powered by a battery.
The circuit above has a signal "sitting" on the TOUCH PLATE via the oscillator made up of a Schmitt trigger between pins 1 and 2. The operates as a square-wave oscillator at approximately 150 kHz. The oscillator's output gets ac-coupled to R2 that sets the drive level and hence, the sensitivity for the touch pad. Applying negative excursions of several volts of a square-wave signal to its gate repetitively drive N-channel JFET Q1 from conduction into cutoff. An approximation of the square wave swinging from 0 to 12v appears at Q1's drain. A peak detector circuit formed by D1; R7 and C4 provides sufficient dc voltage to force IC1B's output to a logic low.
However, if someone touches the touch pad, any added capacitance to ground reduces the ac drive at the FET's gate, and Q1 continuously conducts. The square-wave voltage applied to D1 decreases. The voltage on C4 drops below the logic threshold, and IC1B's output goes high. You can adjust R2 to set sensitivity and compensate for device-to-device variations in the FET's pinch-off voltage.
The following circuit does not work. It uses a CD 4001
The TRUTH TABLE for a NOR gate is:NOR GATE 0 0 1 1 0 0 0 1 0 1 1 0
We can see from the Truth Table that the output of a gate only changes when both inputs are LOW. For the top gate, pin 1 never goes low so this type of gate will not work.
Try a NAND gate:
The circuit above does not work. By checking the Truth Table, we see the gates are correct:But the circuit does not turn off. The reason is the 4u7 is not charge or discharged by any component in the circuit. When the circuit is first turned on, the electrolytic is uncharged and pin 5 is effectively connected to pin 3. If output pin is HIGH, pins 5&6 will be HIGH and pin 4 will be LOW. This will make pin 3 HIGH. Both the Touch Wires will be HIGH and touching them will not change the state of the circuit.NAND GATE 0 0 1 1 0 1 0 1 1 1 1 0
We need a component to allow the 4u7 to charge and make pins 5&6 LOW.
The next diagram does this:
The 100k "safety resistors" have been removed as they do not play a part in the operation of the circuit and the touch wires have been connected to the circuit to have the greatest effect.
CIRCUITS B
The following circuits show a "flip-Flop" effect. The circuit changes state, each time the touch pads are touched.
If a finger is kept on the touch plates in any of the toggle circuits above, the circuit will oscillate ON, OFF, ON, OFF at a low frequency. The frequency of 3 sec, 0.5 sec has been identified in the top circuit. An improvement to the Toggle Touch Switch above, to keep the charge on the 100n, is to use a second gate:
A touch switch can be made with 2 gates from a 4049UB IC, as shown in the following circuit. It has proven to be reliable at 6v and 12v. The design has the advantage that the output does not cycle if a finger is kept on the Touch Pads.
CIRCUITS C
These circuits have two touch plates. One touch plate turns the circuit on and the other plate turns the circuit off. 
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The TOUCH-PADS deliver current from the power rail to the input of the circuit, via a moist finger. The finger acts as a very high vale resistor. Note the 4M7 feedback resistor that keeps the circuit on when the finger is removed.
The circuit above is available from Talking Electronics as a kit. The kit is called TOUCH SWITCH:
TOUCH SWITCH USING A CD 4011 IC
A TOUCH SWITCH using a CD 4011 is shown in the diagram above.
A simpler version is shown below:
When the circuit is first turned on, the two gates will "race" and the fastest gate will create a HIGH output. It cannot be determined if the LED will light when the circuit is first turned on. By adding the 100p (shown in red) to the position shown on the circuit, one input of the gate will start with a LOW and this will make pin 4 HIGH. The top gate will have HIGH on both inputs and the output will be LOW. This will turn on the LED. It is not know why the previous circuit used all 4 gates of the 4011. The circuit was taken from a kit manufactured by a non-electronics person and he did not investigate the possibility of simplification.
Since the output of a CD 4011 is not capable of sinking or sourcing a high current, you can buffer the output of the gate with the third gate in the chip and wire it as an inverter.
ON-OFF TOUCH SWITCH USING A 555 IC
For those who like the rugged 555, we have included a 555 ON-OFF touch switch.
TOUCH PADS
A touch Pad can be obtained from many different sources. The photos below show a touch pad obtained from a toy. Some of the very light touch buttons consist of a small carbon block mounted in silicon rubber and when the button is pressed, the carbon block touches the pad and reduces the resistance between the two interleaved tracks.
3 TOUCH PADS<This part of the circuit board can be cut away and used as a touch pad for the circuits in this discussion. The pads are already protected from corrosion and form a very good design for detecting a finger.
Close-up of the touch pad
The important feature of the pad is the number of interleaving fingers as this is equivalent to a pair of lines about 12cm long and when a finger is applied, the resistance between the lines drops to between 150k and 850k, depending on the pressure and moisture in the finger.
HIGH IMPEDANCE CIRCUIT
We have already said a touch pad is a high impedance device (circuit), but what does this mean and how does it work?
We are going to explain why it must be a high impedance circuit.
Below we have four different touch pad circuits. The supply voltage does not matter, however we have shown it as 6v. The main purpose of a touch pad is to reduce the voltage on the "output." Generally this must be15% - 25% of rail voltage to trigger the circuit.
If we take the first circuit "A" and place a finger on the touch pad, the circuit becomes equivalent to two resistors in series. These two resistors form a voltage divider and the voltage on the output is in proportion to the value of the resistances. We will assume the resistance of the finger is 1M to make the discussion simple. The 5M resistor is not a standard value but s also used to make the discussion easy to understand. In the diagrams below, the output of the
touch pad is 6v when nothing is touching the pad. When a finger touches the pad, the voltage drops to 1v. Without using mathematics, we can see the 5Meg resistor is in series with the 1Meg finger, making a total of 6Meg. This means 1v appears across each 1Meg and thus the output is 1v.
If we apply the same finger to circuit "B," the output voltage will drop to 3v. This voltage may not be low enough to trigger the circuit connected to the touch pads.
If we apply the same finger to circuit "C," the output voltage will drop to 5.4v. This voltage will not be low enough to trigger any circuit connected to the touch pads. Let's look at how this voltage is created. The two resistors are 100k and 1M in series. If we convert the 1M into ten 100k resistors, each resistor will have the same voltage across it. There are 11 x 100k resistors and this means very close to 0.6v will appear across each resistor. That is why the output voltage will be about 5.4v when the finger touches the pad.
From this we can see the "pull up" resistor must be as high as possible so the effect of a finger will reduce the output voltage of the pad to a low value.
There is one other important factor to remember.
The output of a touch pad must be connected to a high impedance input. The diagram below shows the gates and a "super-alpha" transistor. These all have a high impedance input.
Why do we need a high impedance input?
Suppose the circuit we are connecting to the touch pad has a low impedance. It will be equivalent to placing your finger on the touch pads. The output will go low and your finger will not be able to create a HIGH-LOW voltage change.
The input impedance of a gate can be considered to be very high (greater than 10M). When the "super-alpha" pair is connected to the touch switch, the voltage on the "output" of the touch pad will not rise above 1.3v. This is due to the base-emitter junctions of the two transistors.
The output of the super-alpha pair will be low. When a finger is placed on the touch pads, the output of the super-alpha pair will rise.
An alternate circuit for connecting touch pads to a super-alpha pair is shown below:
LATCH CIRCUIT
Here are two latch circuits using transistors. The first operates exactly the same as the 4-transistor Touch Switch above. It can be used with a touch pad. It's another "Building Block" to add to your collection. The second circuit operates in the same way. When the circuit is first turned on, both transistors are not conducting. As the input voltage increases to 0.65v, the BC 547 transistor turns on and this turns on the BC 557. The BC 557 is connected to the base of the BC 547 and it takes over from the input voltage. The two transistors turn each other on until both are fully turned on. The supply must the turned off to reset the circuit.
Here is a Touch Switch circuit from a magazine:
Why use half a chip and a FET to do the same as our 74c14 circuit above?
That's why you need to know how to design circuits, so you don't over-design.
See our "Spot The Mistake" article for more over-designed and incorrectly designed circuits. You learn more from other people's mistakes than anything else.
USING A TOUCH SWITCH IN A PROJECT
1. DOORKNOB ALARM
The 74C14 (40106) is a hex Schmitt trigger IC with 6 gates that can be used for 6 different building blocks. Even though it has a "74" marking, it can be placed in a circuit with a voltage as high as 15v - all the other 74 series require a maximum of 5v for the supply. (More data on the 74C14 can be found in Chip Data eBook.)
In the following circuit, the gates are used to detect the touch of a door knob and produce an output that goes HIGH for approx 1 minute.
The output of the above circuit can be taken to an alarm. Open the reed switch contacts and connect the reed switch to the output of the Door-knob alarm.
A suitable alarm can be found in the $2.00 "Junk Shops" for about $2.00 These consist of a piezo diaphragm and a driver circuit consisting of a transistor and COB (Chip On Board) to produce a very loud wailing sound. Some of the devices have an inductor to increase the voltage to about 60v to 80v to produce an output of about 90dB. The device we bought had a transformer to drive the piezo to 80v.
The photo shows the device and magnet. The magnet holds a reed switch closed and when the two items are parted, the reed switch opens and sounds the alarm.
The reed switch can be seen in the photo below. It is an uncovered reed switch consisting of two soft-iron strips that overlap slightly in the centre. When a bar magnet is brought near, the two strips become magnetised with each forming a north at the top and south pole at the bottom. This means the top strip has a south pole at its bottom and the lower strip has a north pole at its top. Since unlike poles attract, the two strips will touch each other when a bar magnet is present.
When a magnetic object comes in the vicinity of a magnet, it becomes temporarily magnetised with North and South poles. This is shown in the diagram. This is how the two strips of the reed switch close and "stick together" when the magnet is near.
The magnetic field of the bar magnet causes the two
parts of the reed switch to become "magnetic."
The side of the alarm showing Chime (Doorbell), Off
and Alarm. See below for a link to these sounds.
The underside of the alarm showing the COB module and the 4 pins from the transformer that drives the piezo diaphragm. To hear the "DoorBell" sound and "Alarm" sound, click HERE. or here: SOUND
Open the reed switch so the Door-Knob circuit can operate the alarm.
2. TOUCH MOTOR CONTROL
- by L. W. Brown, Burwood, Victoria, Australia.
The following circuit is suitable for operating a12v motor such as on a display in a shop window. The 50mm x 50mm touch plate can be stuck to the inside of the glass and anyone placing their finger near the touch plate (on the outside of the window) will prevent the signal entering the charge pump section of the circuit and keeping the 10n charged.
The circuit will take a few seconds before the 10n is discharged via the 10M and the motor will operate.
3. TOUCH-ON TOUCH-OFF
This circuit is an extension of the Door-knob Alarm presented above. It turns on an output when the Touch-Plate is touched very briefly and turns off the output when the plate is touched for a slightly longer period of time.
This article has covered more than 10 building blocks and shown how to adapt a low-cost item in a junk shop to a circuit you have already designed.
TOUCH-ON TOUCH-OFF SWITCH
It has also covered the concept of a HIGH IMPEDANCE CIRCUIT and FEEDBACK to keep a circuit stable in either of its two states.
Even if you think you will never need a TOUCH SWITCH in a future project, the building blocks we have covered can be used in lots of different circuits and if you build them, you will have a much-better understanding of how they work.
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