FM Bug

S.C. Dwivedi

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This FM bug transmitter circuit will let you spy on people. The transmitter can be placed in the desired room and the conversation heard from a place far away just using a regular FM radio set.

The circuit is designed around a single transistor 2N3904 (T1), a custom-made coil (L1), three capacitors (C1 through C3), a trimmer (VC1), two resistors (R1 and R2) and, of course, a condenser microphone (MIC1). The circuit transmits in the frequency range of 88-105 MHz. Transmission range is 100 metres.

Working of the circuit is simple and based on analogue modulation in which a carrier signal is varied corresponding to the message signal.

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The microphone picks up the sounds in its vicinity to produce corresponding electrical signal. This is the message signal that needs to be transmitted over FM band. The message signal is fed to the base of transistor T1. The tank circuit made using trimmer VC1 and coil L1 generates the carrier frequency. This frequency can be tuned using the trimmer. Transistor T1 modulates the audio signal from condenser microphone over the carrier signal produced by tank circuit. This modulated signal is transmitted through the antenna (ANT.).

Using trimmer VC1, tune the carrier frequency in FM band and confirm it with an oscilloscope. You will hear the conversation picked up by MIC1 when you tune frequency of the FM radio set to match frequency of the carrier.

Prepare the coil L1 using about 25cm length of 25SWG wire. Wrap the wire around a cylindrical object of 6mm diameter and take it out after eight turns.

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Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. After proper soldering, fix switch S1 on the front side of the cabinet. Ensure that the oscillator is properly tuned. Addition of a dipole antenna will extend range of the FM bug transmitter. The circuit works off a 3V battery.

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