Mains Voltage Detector

The detector is intended to sense and signal to another circuit that an appliance is connected to the mains voltage. For this purpose, an optocoupler, IC1 in the circuit, is used. The light-emitting diode in this device is connected across the mains voltage rectified by bridge B1. The mains voltage is applied to this bridge via potential divider R1-C1-R2. When the capacitor has a value as specified in the diagram, the current through the diode is about 700 µA (for a mains voltage of 230 V). This results in sufficient light to make the photo-transistor conduct. The drop across the LED is about 1V.


The detector draws a current only when the monitored equipment is switched on. It is intended to be built into the appliance whose mains connection is to be monitored and must, of course, be connected behind the mains on/off switch. A possible application of the detector is in the preamplifier described in this blog (‘DIY: From vinyl to compact disc’). When it senses that the record player is being switched on, it can be used to link the Line-In input of the soundcard automatically to the preamplifier. Another possible application is its use as a power-on reset circuit in a protection system.


Transistor T1 can switch currents of up to 10mA; in the prototype, the knee voltage of the transistor was around 200mV at a current of 20mA. The maximum permissible switching voltage of the optocoupler is 30 V. Fuse F1 is added to allow a fuse to be omitted on the monitored appliance.


Resistors:

• R1,R2 = 100Ω
• R3 = 100kΩ
• Capacitors:
• C1 = 10nF 250VAC (class X2)
• C2 = 47µF 25V radial

Semiconductors:

• B1 = B250C1500
• T1 = BC547B
• IC1 = CNY65

Miscellaneous:

• K1,K2 = 2-way PCB terminal block, pitch 7.5mm
• F1 = fuse holder with fuse (rated as required)