Flowing my previous topic about people meddling with my car and wanting a deterrent I am still suspicious, Changed the oil today and the sump plug was loose if not part undone, never happened before. I also have a nail in the back tire on the offending side of the car the MOT test told me. So plan is that I'll set up a light in the car so that anyone touching it will set it off.
I can preferably use some sort of touch sense or have to resort to pir sensors.
Anyone any idea on how to achieve the touch method ? I assume that it's a capacitive measurement ? Once again the problem of needing to use the physical ground as "earth" for sensing a common side for vehicle and "toucher" (scumbag)
Hello Simon
I don't know if it will help with your car application but I've attached a schematic, pcb and parts list for the touch switch I use in my LED lighting system. Maybe you can substitute the car for the pcb circle area.
You can consider a capacitive transducer that uses the car body, with a single insulated chain to provide the ground reference. otherwise the PIR detectors will work as well.
what do you mean by insulating chain ?
The PIR sensors will be a pain in the ass so I'd rather opt for capacitive if i can
You need a camera pointed at the car.
Well they are cowards so the risk of being discovered will curtail them. They only do stuff to the side of the car that is obscured. if a whopping light goes off in the car illuminating it and the surrounding area the cowards won't want to stick around. If they persist in touching the car I'll make the horn go off.
They'll learn and figure it out and it will become an endless game of cat-and-mouse. You need something that results in discovering their identity and reporting the bastards to the police, not something that entices them to play a game of "how do I screw with his car without him knowing?".
I don't remember the magazine or any details but i'm sure there was a circuit in a magazine many many years ago for detecting if someone touched your car by measuring the car to ground capacitance/inductance and monitoring for a sudden change.
I have no idea how well it worked though.
This could have been 20+ years ago
Have you tried hiding in the bushes with a Jason mask and chainsaw?
Well they are cowards so the risk of being discovered will curtail them. They only do stuff to the side of the car that is obscured. if a whopping light goes off in the car illuminating it and the surrounding area the cowards won't want to stick around. If they persist in touching the car I'll make the horn go off.
I bet they cannot see in the infrared spectrum, put some infrared leds and modified webcam if you can't get anything else. Webcam+infrared+motion detection with software. Unless those cowards are using nightvision or their cellphone cameras to check the surroundings. Also you didn't make it clear did you want to catch them, or see if the car has been tampered with, or just to scare them off?
However I can find perfectly reasonable explanations for loose sump plug and nail in a tire. Unless you didn't drive on the road at all, those kinds of things happen all the time.
I hate thiefs.
I wish it was legal to just supply a crap load of volts to the ground plane of the car, then that way, when they touch it, they instantly vaporize and kill those suckers.
As said by the previous poster, there's no way to stop them. The best way is to get a garage. Other than that, any type of an alarm type system will just make them want to do it more, just to see if they can do it while bypassing your security measures.
You can always shoot them, pull them into your house, and say you were afraid of your life when you heard a noise and found them creeping around inside your house.
Thiefs and vandals suck.
So far it seems they are trying to do things out of idleness and think it is clever lossen things that will fall off later. Yes sump plug could be a coincidence, nail too and I'm willing to accept either but added to the loosening of a tire valve and theft (after much tampering with) of a window wiper I can't call them all accidents. All these things happen down the same side of the car, sump plugs have never come loose on me and I do my own oil changes.
So what ever they do takes time as they are trying to sabotage the car so if they are seen doing it (on an estate that is regularly getting pass throughs by police) they will be less likely to try. All of the problems I've had have been down the same side of the car where it is dark and can't be seen from the road.
Electric fence energiser in the boot. Hook one end to chassis, and just have a Bare Wire hanging Out under The Car And Touching The Ground for The Other End Of The Eht. But Run It off Its Own Battery So That You do Not Zap Your Cars Ecu.
It's a shame you couldn't wire it up to a bird scarer, the unecpected bang those things make would make them crap for a week continuously , and let you know theirs some own messing around, give them a base ball bat welcome and job done, I'll think they'd get the message.
Yeah, the dude with the exploding car door handle thief deterrent could never get anyone to take it up
Not something you'd want to forget to disable
Make sure it ignores cats. I have a neighbour cat that likes to sleep on the warm bonnet. 
And rain, stray static charge (if the weather is dry) and other atmospheric effects... having it too sensitive is just as bad.
So far it seems they are trying to do things out of idleness and think it is clever lossen things that will fall off later. Yes sump plug could be a coincidence, nail too and I'm willing to accept either but added to the loosening of a tire valve and theft (after much tampering with) of a window wiper I can't call them all accidents. All these things happen down the same side of the car, sump plugs have never come loose on me and I do my own oil changes.
So what ever they do takes time as they are trying to sabotage the car so if they are seen doing it (on an estate that is regularly getting pass throughs by police) they will be less likely to try. All of the problems I've had have been down the same side of the car where it is dark and can't be seen from the road.
You need to try and get this resolved. It could quickly turn into something verious dangerous, causing death or serious injury. For instance, if they loosen a zert fitting on a brake caliper or loosen the brake line. You won't notice it, until you try and get off the freeway doing 70mph and you have no brakes. I'd set-up a video camera somewhere outside the car aimed at the car to try and find out who is doing it. Then take the video tape to the authorities. It might just be fun and games and mischief for who's doing it, but it could become very dangerous.