Basic car alarm

[Bluehaze]

Here's a very basic car alarm that's easy to fit and very effective. All you need is a two pole automotive relay, a switch and some wire.
The operation is as follows:
Car doors closed and switch is on (armed)
Our thief comes along and manages to open either front door.
Door switch illuminates interior light, as well as energizing the relay.
Contacts close, horn is grounded and now blaring thanks to contacts 30-87.
Quick, close door. Too late, contacts 30-87b is keeping the relay energized, forming a "self hold" circuit.
The horn will continue to sound until the switch is open or the battery runs flat.
The only thing is the switch needs to be secreted outside the car (like hidden under a guard).
This might not be remote controlled, have motion, tilt or proximity sensors or rolling digit encoded remotes, or even a flashing LED, but for your average thief, a fairly stand-outish car like an FB or EK with the horn blaring constantly is bound to attract attention, and I'm sure if you're within earshot you can recognize the sound of your old girl when some punk is messing with her!

[Harv]

Very cool, and nice and simple - many thanks for posting. Relays I get... latching always seems to bugger me. Seems half the time I attempt wiring, I manage to let the smoke out .

Cheers,
Harv.