Nine-Volt Battery Voltage Monitor  -  This circuit turns on a LED whenever the voltage of a standard 9v battery connected to the circuit drops below 7.2 volts.  It uses a LTC1440 comparator, which also contains a 1.18v reference diode.  In standby mode, the circuit draws only 4uA . . . [Circuit by David A. Johnson P.E.,  07/09/06]

Under/Over Voltage Alarm on AC Line  -  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 . . . [Circuit by Dave Johnson P.E.,  03/29/09]

Voltage Monitor for a 9v Battery  -  Nine voltage batteries are used in a wide variety of electronic devices.  But, you often don’t know when it is time to change the battery until the device stops operating.  The hobby circuit below provides a visual indication when the battery needs to be . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]