I have a 50's classic car and I'd like to fit an immobiliser to the ignition circuit (12v) but I need help with its design.

I'm thinking of a circuit that would control a 12v 10amp relay which interupts the 12v supply to the ignition.

As well as flashing a LED at 5 secs intervals when activated I'd like it to have an inbuilt adjustable timer that allowed the car to start but then after the pre set time, eg 30 secs, it operated the relay and open circuits the ignition wire. The timer would stay in this state for 30 secs before restoring the connection to the relay.

My thoughts are this :

1. the flashing LED makes it look like a standard car alarm / immobiliser
2. the timer function gives the impression that the car is OK allowing it to start but then develops problems and after a few cut-outs the thief will give up and move on as this is an open top roadster with no door locks so it is impossible to stop anyone getting into the car.
3. It also has a mechanical fuel pump so its not possible to cut that off electrically either.

If it could be operated by remote then that would be even better but a simple hidden on/off switch would be OK.

Although I used to do electronics many years ago, I'm totally out of it nowadays from a design perspective but I would be able to construct it from a diagram.

So if anyone could design a circuit for me along these lines that would be really helpful.

many thanks

PS

I found this circuit of a flashing LED. Could this be adapted ?

Attached Images

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Hi there and welcome to Electro tech...

Don't know where you are in the world but this link is for a resonably priced remote control module suitable for the job. if your not in the UK I'm sure it can be sourced someplace else.

http://www.maplin.co.uk/Module.aspx?...=14&doy=search

Keep the led circuit as is flashing on the dashboard just for effect.
It could be cross connected to the remote control module to give the intermittant imobiliser effect but not sure what an insurance company would make of it.

Electronics can work wonders but the defences availible against a determined gang of car thieves are limited. .
... mind wanders back to the open scene of Robocop and the advert for Magnavolt car defence...

on the other hand an old cell phone with a pre-paid sim card makes for a simple vehicle tracking device plus a little tinkering could connect or disconnect a relay in the ignition ciruit.

Anyhow back to driving a relay..
look for a "Solid State Relay" slighty more expensive then a conventional magnetic coil variety but these are designed typically for automotive use and can be driven directly from the 555 output without the need for a transistor of back emf diode configuration.

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It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

 

Back "in the day" I wired several kill switches on vehicles, usually just ran a wire from the neg of the coil to ground. You don't mention what type of vehicle or ignition system, so that may not be the best route for you. The wire from the neg coil terminal,when shorted to ground would replicate a bad set of points. The switch to do this was usually camoflaged in plain view. On one car I gutted the cigarette lighter and put a normally open pushbutton in the bottom of the lighter socket. When the cigarette lighter was pushed in, it pressed the button ,which in turn grounded the coil.Another was a switch mounted to the floorpan under the shifter boot, you had to know exactly where to push on the boot to start the car. Use your imagination..

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gerty

 

Hi thanks for the Maplin link, sounds like what I need and I'm in the UK so getting one of them isn't a problem.

This sounds interesting, can you tell me how its done ?

can you get DPDT solid state relays ?

 

Yes I used to do this myself many years ago, so I suppose if I don't go for something more high tech I could just go for this simple solution along with a DPDT switch to activate the flashing LED.

 

Have not come across a DPDT solid state relay, at least not yet.

"Gerty" makes a good point about older ignition systems, shorting the coil using the NC relay contact makes for a less obvious immobiliser as a smaller conventional or Solid State relay. Plus the smaller current involved means a cheaper relay can be employed.

As for the cell phone tracking it's a fairly basic system, the phones service provider knows what cell(s) the phone is connected or near and can work out a rough geographical location. The cell-phone has to be switched on and getting a signal but a call does not have to be in progress.

.I can hear the tinfoil hats rustling lol

Lot of firms have sprung up offering the service for tracking vehicles, employees and children. Rates vary but I recall the NSPCC was offering a free 10 tries demo (required credit card verification then cancellation, otherwise they would start billing for service)

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It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

 

Ah so what you are saying is when you park up leave a pre-paid mobile phone switched on hidden in the car and then if someone nicks the car the phone can be tracked and its location obtained ?

That sounds simple enough. I might give it a try tomorrow and see if they can track me

 

That's right, any old cell-phone , a pre-paid sim card .. does not even need to have any credit on it. Accuracy varies depending on the cell coverage density, out in the sticks the error was quarter mile, in town this dropped considerably. It falls way short of GPS but is heck of a lot cheaper.

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It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

 

I wouldn't short the coil - thats just asking for trouble.

For relay - have a look at the proper automotive type - for the cost of a few pennies extra (or less if you look around) you can get a good 30A-40A 12V rated Automotive relay. Yes they are SPDT or SPST but you only need to interrupt one line to the coil.

Oh - did I mention I sell them on my website ? The username + .co.uk should get you to my homepage

 

You normally have to get the cellphone "owner" to agree to being followed first.

When you sign up the tracking service sends a text message to the phone. That has to be replied to before the tracking can be done.

The accuracy is not good. We had some showing positions that were about 2 km off.

To find a vehicle, you need a second phone that can be tracked. You then drive to where the target vehicle is supposed to be, and you track the known position. If that is off to, say the south-east, move north-west to get the tracked position of the known phone to match the tracked position of the target vehicle, and you should be quite close.

<blantant plug>
GPS systems are not that expensive any more, and some don't have a subscription. www.on-trak.co.uk
</blatant plug>

 

If you build it they will come...

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It may seem like a good idea at the time , but never stir your cold coffee with a soldering iron.

 

It's probably not going to help much finding your car a few hours or days later. They generally make good use of their time after they steal a car.

You can use the mechanical fuel pump (and presumably carburetor) to your advantage. Put a solenoid in the fuel line before the pump. The car starts normally, but if the solenoid isn't activated the bowl isn't replenished. Thief gets maybe 100 meters and it sputters and dies.

Use something common like the radio controls, running lights, or the cigarette lighter to enable the fuel supply.

 

Do you have a source for automotive solid state relays? They must be new. The SSRs I am familiar with use an internal Triac. They will not switch off after switching on DC current.

Bob

 

Have a look for "high side switch" on the ST website (www.st.com)

I personally wouldn't use a solid state switch on any vital car systems i.e. ignition where failure of the switch could result in the car cutting out unintentionally.

 

In my younger days, my car was protected by an extra ignition coil mounted under the seat. A changeover switch connected the wire from the points to the extra coil, the output of which was bare copper wire stitched into the seat covering while the seat back had earthed wire stitched into it. The switch also energized a flashing LED to remind ME to switch of the "immobilizer".

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A conclusion, is the point reached when you stop thinking.