Single relay car alarm

[Mad Mart]

No Mahesh, read the posts above carefully. The diode is there to keep the relay coil energised after the door has closed again.

[GTS290N]

Once activated the relay contact acts a hold-in circuit and the only way to switch it off is by opening the hidden "Your Switch".
Unless you pop the bonnet, isolate the battery and then nick the car.
Or smash a window, climb in through it and nick the car.
On an old car like ours I would simply wire in a hidden switch to the coil LT, if they're going to nick it there's nothing you can do.
I had my '88 Accord nicked in '93, they jemmied the door smashed the steering lock and hot wired the ignition.
There's a spate of thefts of moderns in Coventry at the moment, they program a key and drive off.

But as an exercise, the circuit Mart and Tony have come up with should work.

[Larry 1300]

Hi and thanks for your help with this. I am working the next few days but will try out the alarm as you have suggested and let you know how I get on'
I see from the diagram that the wire from the interior light doesn't get attached to the relay, just with a diode.
The siren has its own 12 volt power and this is attached to the toggle at one side and to relay 85. Its the black wire from the siren that I have to connect to 86 along with the diode. The cathode end of the diode is to be at the interior light end.
The other side of the toggle has its 12 volt supply also.
Should I put in any in line fuses please?
This is the first time I have wired a diode or a relay.

[soe8m]

I'm afraid that the damage caused by the one who turns your alarm off after three minutes of siren whining will be more than the damage caused by the thief getting in.

Jeroen