When working with high voltage circuits is useful to have a non-contact voltage detector also known as tester which enables to detect the presence of voltage without direct contact. This post presents circuit which allows to determine whether the AC line is live without electrical contact with it. The LED will blink and piezo buzzer will beep when antenna is brought close to the live AC line. The copper wire in the shape of a spiral can be used for antenna.
|Power supply (9V)
|Copper wire spiral
Circuit is based on BC547 transistors. The copper wire spiral (antenna) is connected to the base of Q1 transistor. A small current is inducted when antenna is near to the live AC line. This current triggers Q1 transistor. Signal from Q1 transistor goes to Q2 transistor base and signal from Q2 transistor to the Q3 transistor base. The Q3 transistor acts as a switch which opens circuit. Then LED will start to blink and piezo buzzer will start to beep.
Circuit is designed on a breadboard.