I've been thinking of an inconspicuous way to put a homemade security system on my car... I have an idea, but I'm not sure how the circuit would be made.

In a nutshell, I want to have a circuit so that when 12 volts is applied to it (like from a keyless entry remote), it trips a relay (I guess?) to make or break the connection to the ignition, so the car cannot be started. In the process, I want to have a little LED in there to tell me when the system is armed. Any suggestions?

 

Here's a simple circuit that make the car think it's in park (or you clutch isn't pressed in). You could mount the magnetic switch behind something. to start the car you would would have to have a magnet over the switch (but only you know where the switch is) The light is on, indicating the car won't start if the magnet isn't in place.

That should get you started

 

Oops, I forgot to ground out the B terminal to the chassis!!!

 

why reinvent the wheel? you can get complete keyless entry systems with ground when armed output (starter kill) for around $25 on ebay.

p.s. most alarm manufacturers don't design their systems from scratch either. they instead construct their systems around premade ICs such as the MicroChip KEELOQ security chipset.

 

86IROCZ, thanks for the schematic. Actually, I was going to do identically what you've shown there. The reason I wanted to use a remote keyless entry system was so that a passenger wouldn't wonder what I was doing before I started the car... even with an inconspicuous reed switch, I'd have to move my arm somewhere "odd" to position the magnet.

2vmodular, what are the chances of someone getting around a manufactured keyless entry system? The reason I was designing the circuit myself was so that a theif couldn't easily see what type of security system I was using and then reverse-engineer it to steal the car.

 

most all recent car alarms use a rolling code setup. chances of a thief electronically defeating one of these setups without locating the alarm brain or splices into the vehicle harness is slim to none.

problem is with the install. circuit city, best buy, etc all install the alarm in the same place, under the drivers side dash. once a theif locates where the starter interrupt is spliced or the power leads to the alarm brain are, your car is as good as gone.

 

I agree, intallation is the key. Thieves know where Circut city will put your new alarm and look there first. They'll also notice fancy colored wire ties, wires that arn't in a housing and all the other crap that does't look stock. Hide that system well and don't take any short cuts.

I would just tell the passenger that the magnet was a special chip with a code in it .

Is there any particular reason that your passengers would want to steal your car anyway?

 

Thanks for the tips on hiding the system; I wouldn't let Circuit City (or any store like that) touch my car... I'm afraid to leave it at the garage for an hour, even if I'm just next door in the waiting room. Guess I'm paranoid

Well you know how word gets around... sometimes your best friends inadvertantly or accidentally tell someone something, then it just keeps spreading... I'd want to keep it as low-key as possible.