This simple Voltage Monitor device has two light-emitting diode (LED) indicators that switch on if the monitored supply voltage falls below separate threshold levels. The obvious application is in battery operated equipment where erroneous results could be obtained if the battery potential falls below a critical level.

Having twin threshold levels is very useful, as one can be set slightly above the critical voltage, and it will then give a warning if the battery will soon need replacement. The circuit can also be used with mains powered equipment to monitor the DC supply voltage, and it will then give a warning if the supply voltage drops to an inadequate level due to a malfunction.