This circuit is sensitive to low frequency electromagnetic radiation and will detect for example hidden wiring or the field that encompasses a transformer. Pickup is by a radial type inductor, used as a probe which responds well to low frequency changing magnetic and electric fields. Ordinary headphones are used to for detection. The field that surrounds a transformer is heard as a 50 or 60Hz buzz.
IC = UA741
Resistors = 22K (2), 10K, 10R
Variable resistor = 2.2M
Capacitors = 100uf, 220uf, 100nf
Inductor coil = 1mH (milihenry)a
S1 = on/off Switch