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FM Transmitter (#2)

The FM Room Bug is a neat project that can be built for under $10. It is an electronic transmitter

  1. ElectroMaster
    The FM Room Bug is a neat project that can be built for under $10. It is an electronic transmitter which broadcasts a signal on the FM Band, with a range of 50-500 feet, depending on the antenna used.

    It is powered from a 9-volt transistor-radio battery, which should power it for about 10-50 hours. Use a bigger battery if want use for longer periods. You might also want to try a 9-volt power supply, but I have not been able to get this circuit to work well with a wall-plug power supply. Remember, this circuit was designed for educational purposes only and never intended for spying. It's purpose is for teaching the principles of FM transmitters.


    Because the RF-inductor L1 is actually part of the board itself, it is critical that coil be aprox. the same size as the one shown on the PCB layout. When mounting C5, only two of the three terminals are used. Be absolutely positive that one terminal is connected to the lonesome solder pad on the PCB, and the other terminal of C5 connects to one of the duo pads. The antenna should be made from a 12 to 24 inch piece of stiff copper wire. A longer antenna will increase the range. You may also want to add a SPST on/off switch in series with one leg of the battery's leads. If you want to house the project in a box, do not use a metal one.

    To use this transmitter, tune a radio to a point on the FM dial where there are no broadcast stations nearby. With the transmitter is on, adjust C5 with a non-metalic screwdriver until you can pick up the transmitter's broadcast on the radio. You now should have a working FM Bug!


    View attachment 37940

    Parts List

    RESISTORS: (All resistors are 1/2 watt, 5% tolerance)
    • R1- 27,000 ohm
    • R2- 56,000 ohm
    • R3- 12,000 ohm
    • R4- 100 ohm

    • C1- 1uF, electrolytic (or tantalum)
    • C2, C3- 470 pF, ceramic
    • C4- 6 pF (or 10 pF), ceramic
    • C5- 1-30 pF, trimmer

    • ANT1- Antenna Wire
    • B1- 9-volt transistor radio battery
    • L1- Center-tapped coil (part of printed circuit board)
    • MIC1- Condenser microphone
    • Q1- 2N3564 transistor (or 2N5225)
    • Printed Circuit Board (2.50 x 1.75 in.)
    • Battery Connector