Low Voltage Alarm

Schematic

Parts

Part Total Qty. Description Substitutions 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

Notes

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