FM spy bugs are cool things and we might have seen many of them used in Movies. But we can actually make one fun FM spy bug circuit using small number of components without spending much money. FM bug circuits should be miniature in size to keep it concealed. The above circuit shows a simple FM bug which was built around a single transistor.
WORKING OF CIRCUIT:
A simple Microphone was used to input the voice signals from the environment in which the bug is placed. The signal from the Microphone is then fed to the base of the transistor through a Capacitor C1 to impeded the different frequency present in the speech signal. The appropriate voltage at the base of the transistor is given by the combination of Resistors R1 and R3.
The variable Capacitor VC1 and inductor forms the parallel tank circuit which will vibrate at a resonant frequency. The VC1 is a variable capacitor which is used to generate the desired frequency. The resonant frequency signal generated by the tank circuit if governed by the equation
f= 1 / 2π√LC
The Capacitors C2 and C3 acts as a decoupling capacitors. Whereas the Capacitor C4 across the transistor serves to keep the tank circuit vibrating since frequency decay may occur due to heating losses. A thin strand of copper wire of length 30 inch can be used as an antenna in this above circuit. This circuit over a distance around 50 feet and only suitable for short ranges.
- Use dead air frequencies for clear reception of the audio signals.
- Changing the value of Resistor R2 will alter the sensitivity of the microphone.
- This circuit was intended to hep building a fun bug circuit and cannot achieve much efficiency like the commercial ones.