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 |
1. The circuit will operate from 9V to 12V.
2. Adjust R2 until the alarm goes off at the correct voltage.