Re: headlight alarm circuit



CM wrote:
> Found this circuit on the internet.. It is intended to be a headlight alarm
> (warning) to prevent one from leaving lights on by mistake.
>
> http://www.eed.usv.ro/misc/mirrors/cc/circuit.htm/0122.htm
>
> My question is, what is the resistor for ? do you need it and do you need
> the diode from the ign aux line? Can't we wire the negative from the buzzer
> straight to ign circuit minus the resistor to ground and diode?
>
> Help me undrstand the purpose of the added parts.
>
> Thanks
>
> CM

Hi, CM. You're probably not seeing it because it's so simple.

Let's ignore the "To IGN-AUX" part of the circuit (which happens when
it's at 0v, i.e. the ignition and aux are off, and the diode prevents
current from flowing). The buzzer BZ1 is a low current device. When
"To Lights" is high (i.e. the lights are ON), 0.7V will be dropped
across the diode, and a volt or four across the 1K resistor, leaving 8
to 11 volts across the buzzer, enough voltage to make the buzzer sound.

Now let's assume the key is in the ignition and it's turned to AUX or
ON. There's 12V there, so 0.7V will be dropped across the diode, and
the rest will be dropped across the 1K resistor. That means there will
be no voltage for the buzzer, and it won't turn on.

Net effect: the buzzer will only turn ON when the IGN/AUX switch is
OFF and the lights are ON. Simple & straightforward.

A buzzer isn't specified. I'd recommend trying your buzzer or binger
with 12V and a 1K series resistor before you install. It's possible it
might not be loud enough for you.

Good luck
Chris

.

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