This Mini FM Bug Detector is basically a broad-band receiver. It picks up the complete band from below 80Mhz to 150MHz and almost anything that transmits in that band will be detected. If a bug with a sensitive microphone is transmitting nearby, the result will be a feedback whistle.
The Bug Detector will not determine the output power of a bug as it is detecting the sensitivity of the microphone.
However it will prove the carrier section is working (this is the section that produces the 88MHz frequency) and also the audio section.
The circuit is basically a broad-band receiver and will pick up all types of transmissions.
The quiescent current for the circuit is less than 2mA and the output is a mini piezo diaphragm. Note: the circuit will not drive a speaker.
Each stage provides high gain and the signal is picked up by an un-tuned front-end. You can see the front end is untuned as the inductor does not have a capacitor across it.
You would think the 20 turn inductor would create a short-circuit to ground. But signals at 100MHz will oscillate in the inductor and can be picked off via the 470p capacitor. Signals of a low frequency will be taken to ground.
The 100MHz signal is amplified by the first two stages and the audio component is detected by the diodes on the base of the third transistor. The fourth transistor is an audio amplifier to drive the piezo diaphragm.
The inductor across the piezo provides a load for the transistor and creates a high voltage during part of each cycle to increase the volume.
The 22n capacitor removes the high frequency component of the signal.
The 47u across the power supply improves performance of the circuit by preventing signals from the output stage passing to the front end.