Posted on Apr 19, 2012 3258
The bug tracer is made up of a simple rf-injector circuit consisting of Ql and Q2, and a pocketsize, AM broadcast receiver. The two-transistor rf-injector circuit supplies a constant rf signal to one end of a cable. Then the AM receiver is used as a detector, allowing you to trace the wire to its source. Transistor Ql, along with piezoelectric ceramic resonator RESl, make up a simple rf oscillator that operates either at or near the AM-radio, 455-kHz, i-f frequency.
That means that the second or third harmonic signal can easily be picked up by the receiver. Transistor Q2 is connected to an emitterfollower circuit to protect the oscillator from output loading; that helps to stabilize the output frequency and signal level.