The circuit depicted is the receiver device of a transmitter/receiver combination that will prove extremely handy when tracing the path of electrical wiring in a building or to locate a break in a wire.
The corresponding transmitter may be found elsewhere in this issue. The transmitter produces a distinctive tone which alternates between 2100 Hz and 2200 Hz.
The matching receiver for the wire tracer is possibly even simpler than the transmitter, as is shown by the schematic. It consists of no more than a short wire antenna (a piece of wire, 10 cm long is adequate), a high-pass filter (C1-R1), an amplifier stage (IC1), an output stage (T1) and a loudspeaker. The prototype used a high impedance loudspeaker from a telephone handset, and this worked remarkably well.
The purpose of P1 is to adjust the amplification. At the highest amplification, the wire energized by the transmitter can be traced from several tens of centimeters away. A direct electrical connection is therefore not required. However, it is important that you hold the ground connection (earth) in your hand.