This low voltage circuit can be used to monitor batteries and other volatile sources of current for problems. The circuit sounds an alarm and lights an LED, but can be interfaced to any number of other circuits for many different uses.

 

Schematic

Parts:

Part	Total Qty.	Description
R1, R3	2	1K 1/4W Resistor
R2	1	5K Pot
U1	1	LM339 Voltage comparator IC
D1	1	1N5233B Zener Diode
D2	1	LED
BZ1	1	Piezo Buzzer
MISC	1	Board, wire, socket for IC

Notes:

1. The circuit will operate from 9V to 12V.

2. Adjust R2 until the alarm goes off at the correct voltage.