AC Line Under/Over Voltage Alarm

Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  This hobby 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.  This hobby 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.

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