At 11:48 PM 5/24/2010, miguel pacheco wrote:
>How about a more detailed explanation of george jannini's dummy alarm?
>Miguel

Hi Miguel,

What you need for a headlight alarm is a buzzer and a diode, to allow
current to flow only one way through the buzzer.  Piezo buzzers are
that way by nature, and the Radio Shack buzzer that I called out
apparently has an internal diode.

Now you must locate two wires:  one of them goes on and off with the
ignition and the other goes on and off with the parking lights.  Hook
the negative lead of your buzzer to the first wire, and the positive
wire to the second.  That's it, you're done.

If both ignition and lights are on, no current flows because the
voltages are equal.

If ignition is on but lights are not, no current flows because the
diode prevents it.

But if lights are on and ignition is not, current will flow through
the buzzer and ground itself through the various devices that are
powered by the ignition wire you chose.  Bzzzzzzzzzzzt.....

Yours,
DAvid