[peppercorn (OP)]

At present, when I'm not using my car (with mechanical points) I take the distributor arm out to stop it being nicked.

I thought:
If I were to purposely offset the distributor - back 20° say, the spark would have no effect.

Then, running the breaker wire through to under the dash I could have an electrical or electronic delay that would 're-allign' the timing - sending the 'off' pulse back to the coil in the right place.

I would then have a handy 'fob' with a push button for starting that I could take with me instead of a key.

I was thinking of adding a PNP power transistor (BD912 from Maplins) to prolong the points, so amplifying will be dealt with.

I know many people just use an isolator switch, but any crook with an ounce of automotive know-how could get round this...

[lyner]

If you changed the ignition to a contactless system, then you might have a chance - else a thief could hot wire it. Moreover, you'll need some hefty circuit components to handle the voltages - like a proprietary device, using the contacts.
You could always fit a tap in the fuel line - that would fox 'em. Or a Clamp!

[peppercorn (OP)]

Tap on the fuel line - why didn't I think of that? !
Trust me to come up with a complicated solution when a simple one would do!!

I'd opt for a solenoid driven valve next to the carb though, or the 'perps' might get half a street away before the fuel died...

W.R.T. my electronic convolution -

Moreover, you'll need some hefty circuit components to handle the voltages - like a proprietary device, using the contacts.

-Not if the inhibition is on the low tension side with a power amp doing the hard work...

[RD]

A car could travel a few hundred meters on the petrol in the float chamber of the carburettor.

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The perps may still smash the window or bend the door to break in to an electronically immobilized car.
So a highly visible steering wheel clamp/lock would be a better option:
 no point in breaking into the car if the steering wheel is immobilised.

This steering wheel lock looks like it could double as an extra large frying pan  []

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[peppercorn (OP)]

So a highly visible steering wheel clamp/lock would be a better option

My bicycle has a quick release seat. I wonder if anyone's ever made a quick release steering wheel!

It'd have to have a safety switch so you couldn't drive off without it being fully engaged on the post, though!!

[dentstudent]

So a highly visible steering wheel clamp/lock would be a better option

My bicycle has a quick release seat. I wonder if anyone's ever made a quick release steering wheel!

All formula one cars have these!

[Chemistry4me]

You mean a steering wheel or the quick release seat?

[dentstudent]

You mean a steering wheel or the quick release seat?

Wheel - haven't you seen a GP?

[Chemistry4me]

Nope.

Remember the last time I talked about F1 wheels? []

[Chemistry4me]

Oh, you mean on TV or live?
TV: Yes. But I've never seen the steering wheel come off.
Live: Afraid not.

[RD]

Even if a steering wheel was removed from an ordinary car
 it may still be possible to steer it with a big pair of these locking pliers ("Mole grips")...

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[Although this would be rather dangerous].

[Karsten]

If you used two it would be like a motorcycle. At least as long as you don't have to turn hard.

My dad had a little hidden switch under the rear seat. It disconnected the ignition I believe. Lift seat, move switch, leave potential thieves puzzled. Don't know if anyone ever tried, but his car was never stolen.

[Chemistry4me]

Whoa! That sounds like a very smart/high-tech car!

[RD]

Evidently one handle is sufficient to steer a car ...

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[lyner]

I just had a vision of everyone wandering around the shopping centre with a steering wheel under their arms (including car thieves - just in case they found an immobiliser which wasn't working)

peppercorn - have you ever measured the peak voltages on the primary side of a car ignition? When you break the circuit, the volts shoot up to hundreds (V = LdI/dt) - hence the beefy semiconductors needed

[Karsten]

Whoa! That sounds like a very smart/high-tech car!

In case this was in reference to my Dad's car: It was an old BMW 316. I am sure he did it with all of his cars though. Can't ask him any longer.

[Chemistry4me]

Ohhhh.... I thought it came with the car.

[peppercorn (OP)]

peppercorn - have you ever measured the peak voltages on the primary side of a car ignition? When you break the circuit, the volts shoot up to hundreds (V = LdI/dt) - hence the beefy semiconductors needed

SC - I must admit I hadn't banked on the voltage climbing that high!
Just to clarify: you've used the formula for an ideal inductor (with inductance L) - Yes?
Can the primary side of an auto-wound transformer be described this simply? With energy transfer to the HT side I would have thought the analysis is more complexed.
I believe the IC I looked up was rated to about 200V for Vce & was similar spec. to ICs used successfully (assuming the homebrew websites I looked at were accurate!).

[lyner]

L dI/dt must apply but it's the dI/dt  that isn't known - the spectrum of the spark (RF) is hard to tell.

I do remember that, when the DIY, capacitor discharge electronic ignition was all the rage, the capacitors used had to be 400V working plus. And that system had a well defined dI/dt.

[Bored chemist]

I did once see a design for a sort of imobiliser circuit using a 555.
The 555 drove a relay that interupted the current to the coil a few times a second.
This made the car misfire and splutter horribly.
It didn't stop someone stealing your car- but it did mean they were not likely to drive it very far.
I'm not sure how this would work with modern electronic ignition and I suspect it might void the warranty.