AC Line Under/Over Voltage Alarm     (March 29, 2009)
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.