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Anti-hijacking car immobilizer

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jjj

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I found a simple enough immobilizer circuit, but I need some additional safety measures:
The circuit shows a basic, automatic immobilizer, which stops car thieves or at least waste their time, searching for ...why the car doesn't start?
Yet, that's not good enough, for if the thieves forced me to hand over the car's keys, I want them to get away for about 30 seconds and then the engine stops. I. o. W. I would need to add a simple delay.
I need some idea on how to momentarily fill the electrolytic condenser, which then gradually bleeds itself out and so, allowing a couple restarts.
I once had such a circuit, which allowed the car to start and run for about 40 seconds and then the engine stalled. The car then could be restarted and run only for about 10 seconds and the engine stalled. The 3rd time it could not be restarted anymore. That was good, because it confused the would be thief and was far enough for my safety, while I'll be busy contacting police.
 
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I prefer mine a stone like RFID hidden in the console , so the keys dont start without the secret stone.
 
A simple capacitor and relay as you showed on another site will not work well for the delay, because the delay will be too short with any reasonable values for capacitance and relay coil current. A better solution is to build the delay with an resistor and capacitor controlling a mosfet switch. In that way, you can use a larger resistor and get longer delays. A member on the other site showed a simulation for such a circuit. Just for reference, here is a similar circuit:
upload_2015-9-30_1-21-26.png


C1 x R1 controls the delay time. The buzzer would be replaced with your relay coil. Also note that this type of the mosfet (N-channel) must be between the load and ground and will need to have a suitable voltage rating for automotive use.

John
 
Thank you jpanhalt for this circuit idea.... Here in St'go de Chile we got drugged 12-year olds "pistoleros" hijacking cars and murdering drivers.. I have only limited electronics knowledge, but need to create such a life saving circuit.
My idea is to peacefully hand over the car keys to a drug addicted car hijacker and just let him know that the car has "mechanical problems..." - the rest then does this hijack immobilizer circuit. I also chassis attached a hidden, locked box in which I hide the car's documents + spare key.
Thus far, I somewhat succeeded in developing it, but there surely are better electronic solutions. In fact a couple of your timer circuits idea might suffice to prevent an engine restart, after engine immobilization?? Maybe you can help me (young pensioner 74) there? Here's my latest circuit idea:
Best.JPG
 
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In Winnipeg and all of Manitoba, Canada, They had shops to install free RF "immobilizers" with key FOB for all old cars without e-locks. It was paid by the public owned insurance corporation, (who by the bye have the Best service and among the lowest rates in the country for same No Fault coverage)

I once moved from there to Toronto here with my mother's old Camry and one day when my son-in-law using the car , it dies , towed to shop and after 2 weeks they could not figure out why it could not start.

The keyfob fell off the ring in his pocket and the "Immobilizer" wired under the hood and behind instrument panel was fool proof.

After he realized it, they asked what to do.

He said "its old, never mind,, I"lol pick it up."

I dropped him off , he drove away,

you should have seen the look on the mechanics face.....
 
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