This circuit is an un-tuned VHF receiver. It is designed to monitor the FM broadcast band and can be carried around the home. Should any unauthorised transmitters be present e.g. FM bugs then the meter will register a strong deflection.
This project has few components, can be made on veroboard and powered from a 9 volt battery for portability. My prototype shown below, worked OK on a Eurobreadboard.Circuit operation is simple. The inductor is a moulded RF coil, value of 0.389uH and is available from Maplin Electronics, order code UF68Y. (See my links page for component suppliers.) The coil has a very high Q factor of about 170 and is untuned or broadband. With a test oscillator this circuit responded to frequencies from 70 MHz to 150 MHz, most of the FM bugs are designed to work in the commercial receiver range of 87 - 108 MHz. The RF signal picked up the coil, and incidentally this unit will respond to AM or FM modulation or just a plain carrier wave, is rectified by the OA91 diode. This small DC voltage is enough to upset the bias of the FET, and give an indication on the meter. The FET may by MPF102 or 2N3819, the meter shown in the picture is again from Maplin Electronics, order code LB80B and has a 250 uA full scale deflection. Meters with an FSD of 50 or 100 uA may be used for higher sensitivity.
|Circuit Exchange International||Return to Radio Circuits||https://www.cxi1.co.uk|