Unique Car Security

Thread Starter


Joined Jun 15, 2007
Im thinking of making some small alterations to my car to avoid it being stolen. Im thinking of using a hidden kill switch to earth the coil so the spark plugs dont ignite, and another switch to simply turn off the electrics of the car. If I use switches with three options I can arrange it so that if the two switches are set to the wrong combination then one or more fuses blow.

Another idea I have is to use some sort of "light dependent resistor?" (or similar, I need advice..) as a switch. So to turn the switch on I would have to either block light or shine light on the "switch". Any advice is welcome, especially regarding specific light dependent devices that you think could work in such a situation. Cheers


Joined Apr 20, 2004
There are many ways to disable a car's ignition. One idea I saw in a magazine used a circuit that would let the engine start and run for 15 seconds and than kill the ignition. A restart would let it go for three seconds. After that, the engine would not start again. The idea was to convince the thief that the car suffered from poor maintence, so he would not lok for a hidden switch.

Disabling the fuel pump would work as well.


Joined Apr 20, 2004
The caveat with that is making it failsafe, so the car is not disabled when you legitimately want to use it. Any circuitry that installs in a car has to be able to operate in a pretty harsh environment. Hiding it is problematic, too as the thief has access to the hood latch and such.

Something simple, like a reed switch contoilled by a magnet might be less obvious. Too bad nobody has those figurines to go on the dashboard with a magnetic base. Just arrange it to diable the ignition if it's moved an inch or two. Perhaps something you poke into a hole in the dash to make a switch. It just has to be something that is not easy to see, like a big toggle switch just under the dash would be.

I've got a new Honda that has an ID chip in the key. The car won't start without it being present in the ignition lock, or so they say. There may already be a way around it.


Joined Oct 14, 2005
As long as the immobiliser is not tied up to the ECU and the EFI, then it's pretty easy to circumvent. This would include any home made or third party immobilisers.

Diesel ones are worse, they are mostly tied up to the fuel valve solenoids, and the only physical protection in this case is a small metal plate on top of the solenoid on the injector to prevent access, which can be pretty easily taken off.

Most commercial systems have fail-safe mechanism to prevent activation when the ECU thinks the engine is on, which is a big gaping loophole :D I think this is safety related, you don't want the engine to cut-off when you are on a busy road at high speed, do you? Just bear this in mind when you design one yourself.


Joined Oct 5, 2008
Have you thought about an RFID system? Put the antenna loop in or under the seat and a tag in your wallet. The output from the RFID board closes a relay that allows the starter circuit to energize when the key is turned. Unless you're sitting in the seat with the tag in your wallet, the car won't start.


Joined Mar 6, 2008
I think that the reed switch idea is pretty good, as it's invisible if you don't know it's there.
What about a circuit that sets a flip-flop if the key and reed switch are on, and resets it when the key is off? That way, you need the magnet there to start the car, but removing it won't stop the car.


Joined Nov 29, 2005
"Earthing the coil" may not be proper in modern computer controller cars, you may fry some computing brains.
Most anti-theft systems do not work in 'hi-jacking' events

The most interesting anti-theft gadget I have known of is a key-released plunger placed under the carpet by the driver's left foot.

When you exit the car, (at gun point or not) stepping on it mechanically shuts a fuel flow valve under the car body, with no suspicious movements.
Car may run a few seconds and usually die in the middle of the street.

Entering the car, fuel flow is enabled with a key that pops-up the out of sight plunger again.

Another technique is focusing in recovery : I once had a hidden CB radio pulsing RF once a minute full time, to facilitate recovering.
Now, a hidden cell phone could silently respond to your call giving away its GPS coordinates. Perhaps that exists. Not sure.

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Joined Oct 22, 2008
When I was in college I had a job delivering bundles of newspapers to a few dozen stores around town. I wanted to be able to leave the motor running during each delivery, so I made a hidden latch down behind the brake pedal that physically blocked the car from shifting out of park. All it took was a little bent piece of steel hinged to the firewall.

When I shifted back into gear, the tapping of the latch was so minor a motion that even if someone was riding in the car, they might not see that I'd done anything, but without tapping the latch the car was completely immobilized. It worked great.

Once in gear, the latch floated on top of the shift mechanism with no effect.

Sure, if someone had the time to look, this would be seen pretty quickly, but the point was to discourage the casual joyrider from jumping in and taking off.

Nobody ever tried stealing it, but I sure felt a whole lot better having my car protected as I did my deliveries .

Steve C

Joined Nov 29, 2008
we usually just used a relay linked to an existing switch in the car. Say, the left turn signal which had to be on to disable the starter kill.

Realistically, most car alarms on the market today include a starter kill. But it really is just as easy as a relay on the starter relay coil.

Everything else is just as clever or unclever as you want to be. I've done the RFID in the wallet, the linking to the armed security system, the left turn signal, folks have asked for special switches hidden in, say, the coin pocket of the car, I've ever seen folks put push on push off switches at the top of the kick panel and tap them with the foot when seated in the car.

My favorite method is via an active RFID purchased from eBay called "K9 sombra". It's an active RFID that resides on the keychain and includes a starter kill. Everything you could want in one package.


Joined Apr 20, 2004
There's the 1950 Buick hidden starter switch - you had to mash the gas pedal down to the floor board to press the starter button.