Motorcycle Low Voltage Warning, by RP.

Parts List:
    U1 = LM339, Op-Amp
    Q1 = 2N3906, PNP transistor.
 D1,D2 = 1N4148, signal diode
   ZD1 = 1N5235, 6.8V zener diode
    R1 = 1K
    R2 = 10K, trim pot
 R3,R5 = 10K
    R4 = 1M
    C1 = 4.7uF, 25V, electrolytic capacitor
Buzzer = 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 begins charging 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 while cranking the starter, but still goes off soon enough to keep you from getting too far from your bike. Thanks R.P!