Preventing driving away with engineheatercable plugged in.

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Thread Starter


Joined Oct 1, 2011
Wondering if anyone would like to be kind and help me.
Have a small project that will prevent driving away with the car when the connector cable for the engineheater is attached to the car.
I work as a car mechanic and been asked by the local firedepartment to come up with a soloution. As they have some additional electrical equipment as radios and so they
also have a batterycharger connected to the engineheatersystem to prevent the battery from being discharged when the car is standing at the station.
Thought something like this:
Mount an extra compartment outlet for interior heater in the car with a 220V ac - 12V dc poweradapter:
When the connectorcable is plugged in to the car the poweradapter draws one relay on a relay card: which cuts the
control current to the car's own relay for the startersolenoid.
I would also like to have a visual indicator on the dashboard that tells you that the connectorcable is attached.
I would like to have a 2-colored LED that flashes red when the cable is plugged in and turns into green for some 20-30 seconds after you disconnected the engineheater and It´s possible to start the engine.
The second relay on the relay card will pull when the engine heater is plugged in and give U-bat to a simple little LED flasher with a 555 timer:

Now for my problem as a novice:
Is it possible to somehow involve an extra capacitor in the circuit that can power the LED´s green section in some 20 seconds when you disconnect the engineheater and the voltage to the timer circuit is broken and if so, how do I connect the capacitor and the two-colored LED?
Sorry for my bad english...


Joined Dec 26, 2010
Automotive projects are not dealt with here. Frankly, I doubt whether any sane Fire Department would entrust such a task to anyone but a properly qualified engineer - not a mechanic with little knowledge of Electronics. In case of an accident, they could find themselves in serious legal problems.

Is this actually a school assignment?

Edit: Imagine the consequences of a failure of your modified equipment, such that the vehicle cannot be started when needed for an emergency call. The result of that could easily be loss of life.


Joined Nov 25, 2009
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