Non-Contact AC Mains Voltage Detector

Radio, 1997, N 3

The circuit diagram of the non-contact voltage detector is shown on the figure 1. It consist of two parts - the AC amplifier and the audio oscillator, based on the Schmitt trigger DD1.1 of IC CD4093 with the network R7C2, which determines the frequency of the audio signal, generated by piezoelectric buzzer BF1.

non-contact voltage detector circuit diagram

Fig. 1. Circuit diagram of non-contact voltage detector.
DA1 - UA776; DD1 - CD4093; C1 - 47 mF; C2 - 33nF;
Op-amp UA776 can be replaced with a 741 (then you don't need R5), but it may reduce the sensitivity.

When the antenna WA1 is located near a power supply, the interference of 50 or 60 Hz is amplified by IC DA1 and as a result the LED HL1 will illuminate. The same output voltage of the op amp starts audio generator.

The current consumption of the device is no more than 2 mA when using 9V battery, and current consumption goes up to 6..7 mA when the LED lights up. If disconnect the LED and use only audio generator then it reduces current consumption.

The PCB is shown on the figure 2. Antenna WA1 is made of a foil strip with size of about 55x12 mm.

PCB for non-contact voltage detector

Fig. 2. PCB of the non-contact voltage detector.

The PCB is enclosed in a suitable plastic box, the antenna must be as far as possible from a hands. The switch SA1, LED HL1 and piezoelectric buzzer BF1 is mounted on the front panel.

The sensitivity of the device is adjusted by the potentiometer R2 To make it smoother the potentiometer R2 220K can be replaced with another one of 22K and a resistor of 200K, connected between the lower pin and the ground.

Correctly assembled device doesn't any special adjustment.

E. Stahov