Radio 2000, 7
Sometimes we need to know the exact location of mains wiring, for example, when we repair a house, to avoid accidental damage. This circuit (fig. 1) can help out. It's based on the three transistors. Transistors Q1 and Q2 are wired as a compound transistor pair, the transistor Q1 is an emitter follower and Q2 is in the common emitter configuration. This is the first stage of the audio amplifier. The second stage based on the transistor Q3.
Q1, Q2 - BC547; Q3 - BC557
The gain can be adjusted by the potentiometer R5. The load of the circuit is the low resistance headphones BF1. This headphones can be replaced with a piezoelectric beeper connected in parallel with a resistor of 1 kilohm. The maximum volume is limited by the resistor R8.
The antenna WA1 is the sensor, it is connected to the input of the amplifier. The antenna is made of a strip of copper wire with a diameter of 0.8..1 mm (AWG 18..20) and with the length of about 0.5 m. Connect a small metal peace to the far end of the antenna, it can improve the sensitivity of the device.
Test the circuit to see if it working by touching the antenna with a finger, and you can hear the humming sound in the headphones, the volume of this sound depends on the level of interference and the position of the wiper of the potentiometer R5.
The same humming sound should appear in the headphones while moving the tip of the antenna along the possible AC mains. The maximum sound volume indicates the exact location of the hidden mains wiring.