This transmitter is like the ones in the movies which are used to spy on people or try to record conversations, well don't use this this to spy on anyone, its just for educational purpose only.
What does this do?
- Well all this is a FM transmitter so you could start your own mini FM station.
How does this work?
Well all of us have heard of "frequency modulation", most commonly known as "FM", this circuit works on the very same principal to transmit audio signals captured by the microphone. This circuit uses BC547 transistor to amplify the signal and then frequency modulate it. Since it is tiny and powered with just 9v the signal range is limited to only 15m.
If you like my project you can vote for me in the battery powered contest.
OK now enough of the talking and let's start building.
Step 1: Tools and Components
- BC547 Transistor
- An microphone
- A variable capacitor 47pf
- An Inductor (see steps for description)
- 4.7k Resistor
- 330ohm resistor
- 1n capacitor (102)
- 10p capacitor
- 9V battery
- Soldering Iron
- An FM receiver (any mobile phone)
Step 2: Components
I had to de-solder the parts of the old board, for those who don't know how to solder and de-solder there is a bunch of instructables that describe how to do this and learning to solder is not a hard task.
Step 3: PCB
Make sure to get a PCB with big holes as the variable capacitor pins won't go in the standard size holes.
Step 4: Microphone
Why not use some wires to hold the microphone?
I would not suggest wires as when you tape the circuit if the last few steps you would not get a clear audio. I tried it and got a lot of noise. I got lesser noise when I used the male pins soldered to the microphone.
Step 5: Circuit
And for the antenna just use a thin 5cm long wire. For more stability you could center tap the coil and solder the antenna to the center tap.
Also If you notice the circuit has a LED in it, it is used to show when the circuit is functional. I did not add the LED in my circuit because it was draining my battery faster.
Step 6: Taping Time
You could also use a heat sink instead of tape, I used tape because I wanted to experiment withe circuit so I did not want it to be permanent.
Step 7: Tuning to the required bandwidth
Use your mobile phone to find the signal Manually tune the variable capacitor to match a frequency
The first step is recommended all you have to do is power the circuit and turn on auto find bands on your mobile. Your mobile would scan for channels and all you have to do is look for your transmitter (play some music in front of the transmitter) on that list.
The second method is time consuming, in this method you have to turn on your radio and the circuit. Keep the radio at a specific channel, and then tune the variable capacitor extremely slowly. When you hear stuff on the radio maybe a song that you are playing stop and the bandwidth on the radio is the required bandwidth.
Step 8: Re-Chargeable version
The rechargeable batteries provide longer transmission than the common 9v battery.