This is a
design circuit for non-contact AC power monitor for
home appliances and laboratory equipment that should remain continuously
switched-on. This circuit is built around CMOS IC CD4011 utilizing only a few
components. NAND gates N1 and N2 of the IC are wired as an oscillator that
drives a piezo buzzer directly. This is the figure of the circuit;
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 piezo buzzer 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.
