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Power lines, which deviate much beyond normal voltages can damage expensive
electronic equipment. The circuit below sounds an alarm whenever the line
voltage is higher or lower than normal. I set the alarm limits at about
+-15% from standard levels. The circuit rectifies and filters the power line
signal. I set the resistor values, so the DC voltage produced is close to 1%
of the RMS value of the line. Thus, a 120vac line would yield about 1.2v DC.
That voltage is fed to a pair of voltage comparators.
The outputs of the comparators decide if the sampled voltage is above, below or
acceptable. Three LEDs indicate the voltage range. A green LED is lit if the
voltage is OK. If the voltage is outside normal levels, the circuit sounds an
alarm using a small magnet or piezoelectric beeper. I have shown recommended
resistor values for both 120vac and 240vac power lines.
Click on Drawing Below to view PDF version of Schematic
