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

R1,R2 = 100Ω
R3 = 100kΩ
C1 = 10nF 250VAC (class X2)
C2 = 47µF 25V radial
B1 = B250C1500
T1 = BC547B
IC1 = CNY65
K1,K2 = 2-way PCB terminal block, pitch 7.5mm
F1 = fuse holder with fuse (rated as required)

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