Non-Contact Power Monitor


Here is a simple
non-contact AC power monitor for home appliances and laboratory
equipment that should remain continuously switched-on. A fuse failure or
power breakdown in the equipment going unnoticed may cause irreparable
loss. The monitor sounds an alarm on detecting power failure to the
equipment. The circuit is built around CMOS IC CD4011 utilising only a few components. NAND
gates N1 and N2 of the IC are wired as an oscillator that drives a
piezobuzzer directly. Resistors R2 and R3 and capacitor C2 are the
oscillator components. The amplifier comprising transistors T1 and T2
disables the oscillator when mains power is available. In the standby
mode, the base of T1 picks up 50Hz mains hum during the positive half
cycles of AC and T1 conducts.

This provides base current to T2 and it also conducts, pulling the
collector to ground potential. As the collectors of T1 and T2 are
connected to pin 2 of NAND gate N1 of the
oscillator, the oscillator gets disabled when the transistors conduct.
Capacitor C1 prevents rise of the collector voltage of T2 again during
the negative half cycles. When the power fails, the electrical field
around the equipment’s wiring ceases and T1 and T2 turn off. Capacitor
C1 starts charging via R1 and preset VR and when it gets sufficiently
charged, the oscillator is enabled and the piezobuzzer produces a shrill
tone. Resistor R1 protects T2 from short circuit if VR is adjusted to
zero resistance.

The circuit can be easily assembled on a perforated/breadboard. Use a
small plastic case to enclose the circuit and a telescopic antenna as
aerial. A 9V battery can be used to power the circuit. Since the circuit
draws only a few microamperes current in the standby mode, the battery
will last several months. After assembling the circuit, take the aerial
near the mains cable and adjust VR until the alarm stops to indicate the
standby mode. The circuit can be placed on the equipment to be
monitored close to the mains cable.


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