fm bug

Posted on Feb 7, 2014

A simple little three-transistor FM wireless microphone / room bug that can really throw-out quite a signal. With a wide tuning range from 80 MHz to 120 MHz, this little device can transmit anywhere within the FM broadcast band (88 to 108 MHz) and can be heard with any standard FM broadcast receiver. A simple modification (described below)

fm bug
Click here to download the full size of the above Circuit.

allows the mike to operate above or below the FM broadcast band. Transmission is line-of-sight, so radius of coverage and be anywhere from 300 feet (indoors, with 9" wire antenna) up to 1 km or more using a properly matched 1/4 wave monopole ground plane antenna on a rooftop in the open countryside. For best results, I built this project on a 2. 5" x 3. 5" printed circuit board. Schematic diagram, printed circuit pattern and component layout diagram are shown below. Warning: This device is for educational purposes only! It is NOT to be used as a bug. Please keep experiments legal. It is your responsibility to check with local law authorities to determine the legal implications if you use this device. Information on this page is provided as a courtesy for educators, students, and hobbyists. Feel free to printout digrams and parts list for you own use. This page isn`t going anywhere, so you may bookmark this page for later use if you have a del. icio. us account. Note: This microphone was built as part of a school project when I was a kid, which was FCC Part 15 compliant at the time. This may or may not be the case today. I no longer have the project with me as it had been damaged (connected to an HV power supply in error, accidentally "frying" the parts with 200 volts DC!). For best results, the FM wireless microphone should be built on a printed circuit board using the pattern shown above. Keep connections as short as...

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