If you want to drill a hole in the wall then you must be sure that there's no electrical wiring. This simple device. shown on the figure 1, can detect electrical wiring in the walls or ceiling. Resistor R1 protects IC CD4011 against electrostatic. A rigid copper piece of wire (~ 18 AWG, with length of 5..15 cm) works as antenna. Sensitivity of the circuit depends on the length of the antenna. When the antenna is placed near the electrical wiring, then the circuit produces a sound with 50 or 60 Hz frequency.
Fig. 1. Simple Non-Contact AC Mains Voltage Detector
This device can detect broken wires in cables - near the broken wire the sound is off. The piezoelectric speaker HA1 is connected to the bridge circuit to achieve higher loudness.
On the figure 2 is shown more complicated circuit, except audio it has visual indication based on LED VD1. Resistance of the resistor R1 should be more than 50 Megohms.
LED VD1 doesn't have a resistor in series with it, because IC CD4011 can limit the current.
Radioamateur, 1998, N9,