RF sniffer circuit

The circuit responds to RF signals below standard broadcast bands and above 500MHz, providing visual and audible indications when a signal is received. By adjusting the bias of the pick-up diodes D1 and D2, this circuit can be used to receive low power signals as well as strong energy sources. Carefully adjust R2 until the LED starts to glow and emit a faint sound produced by the piezoelectric sounder, and you have a very sensitive setting.