Spy On Your Friends!

Listen while you read: The Black Keys - The Wicked Messager


The kit I'm posting this week is a 3 stage FM transmitter you can use to spy on...umm rather, monitor auspicious noises. Its called a 3 stage transmitter because it separates the 2 amplification and oscillation stages into separt parts of the circuit. The first stage, isolates the microphone from the base voltage of a simple BJT transistor. The second, oscillation stage, generates a RF carrier wave. In FM, frequency modulation, transmission the input signal from the microphone is transposed on to a carrier signal. The carrier signal is what you tune your radio to listen to. Look at the picture below to see the difference between FM and AM modulation.

With AM, an increase in amplitude of the input signal results in an increase of amplitude for the final composite wave. Just the opposite is true for FM, an increase in amplitude of the input signal results in an increase of frequency for the final composite wave.

Our final and third stage is another amplification stage. This time it is with a special RF transistor along with a RF choke and a capacitor in parallel to reduce harmonics.

I put this project together back a few years ago and it was probably what inspired me to get into RF electronics. You can learn a lot by doing a project like this one, really how much you learn it is up to you. Since building this transmitter I have built two more sophisticated transmitters that both transmit in stereo. This one however, is a fairly quick build and should only take you a few hours. The range is pretty descent if I remember correctly, within a legal range inside the US obviously. Good luck with the project as always if you have questions you can email me at kurf@thewireup.com.

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