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.
R1, R3 2 1K 1/4W Resistor
R2 1 5K Pot
U1 1 LM339 Op Amp IC
D1 1 1N5233B Zener Diode
D2 1 LED
BZ1 1 Piezo Buzzer
MISC 1 Board, wire, socket for IC