Parts List:U1 = LM339, Op-AmpQ1 = 2N3906, PNP transistor. D1,D2 = 1N4148, signal diode ZD1 = 1N5235, 6.8V zener diodeR1 = 1KR2 = 10K, trim pot R3,R5 = 10KR4 = 1MC1 = 4.7uF, 25V, electrolytic capacitorBuzzer = 12V, 4mA
This circuit is connected to your motorcycle.It’s tapped into a circuit thats only powered with the key “ON”.If you turn the bike off with the kill-switch and leave the key on, as you know, the headlight will stay on and quickly drain the battery.
With this circuit, once system voltage is down a volt and a half or so, the first comparator turns on Q1.Q1 beginscharging capacitor C1.After about 6-8 seconds, the voltage on C1 equals the reference zener voltage and the second comparator turns on the beeper.This way the low voltage condition must be sustained, i.e. won’t start chirping whilecranking the starter, but still goes off soon enough to keep you from getting too far from your bike. Thanks R.P!