Hi Gorgon, and b1mmuo27,
I also got the impession that the poster was describing a arrangement
with two press buttons, one to engage, the other to disengage.
But i wasn't sure.
And the more times i read through that post, the less i understood it.
After reading it many times, i still wasn't sure.
I'm still not sure if thats what is meant or not.
Assuming that it is, then i will offer this possible solution.
Its just a suggestion, i haven't made it.
The values are just tentative, you would have to see what works best.
The circuit is very simple, it relies on an SCR as a latch.
Almost any relay with a fairly low current coil would do although it
was intended for the newer type of reed switch with a NC contact,
these are now common, yet for years they were available but rare.
I assume the extra contact is brass or some non ferrous material.
The R1 resistor is to limit the surge through the SCR, maybe 500
ohms or so would do for a start ?
The R2 resistor is to keep the gate shut except when operating,
maybe 1k would do ?
The R3 resistor has to be capable of operating the gate, maybe 20k
or so ... ?
The SCR is just a normal sort of thing, see what the shop has got,
they are fairly cheap, i would expect 1 amp or so at a couple of
hundred volts working to be normal stock. Its best to go over the
top on the spec for use in a motor car, it won't need to pass an
amp, and the kick-back voltage from a reedswitch is typically low
probably cos they aren't particularly inductive like a relay with
iron bits around it.
SCRs are quite responsive to even tiny spikes. You may need to do
something to stop it responding to spikes from the vehicle.
Maybe clamp it with a twenty or thirty volt zener to soak any spikes
that might come along. Vehicle wiring can be very harsh.
This circuit is offered only as a suggestion, it is where i would
start if i wanted to do what you described. Where vehicle wiring is
concerned my policy is to keep things as simple and minimalist as
possible.
Best of luck with you project, John