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I'm after a circuit that stays on after being temporarily actuated, but when the power is turned off, reverts to the off state. When turned on again, it is still off, but goes on when re-activated. It's for a car immobiliser system, wired into the acc switch on the ignition. I'm assuming a bistable flip-flop could be used, if it was powered from the acc circuit? Or would some form of relay be best? (it has to drive external powered components). I want to activate it with an rfid tag.

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If it has to drive external components and there is some power involved, for simplicity I would consider a small latching relay circuit like this: -

http://www.the12volt.com/installbay/uploads/E33_Latching_Relay_2.gif

The "stop" switch is of course not needed and can just be wired short. You can use a P channel MOSFET in place of the start button.

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  • \$\begingroup\$ Ah, gotta love the good ol' stick circuit (that's the word for such a thing, it hails from railway signaling where this stuff was quite common to give a relay-based signaling circuit a few bits of memory, so to speak.) \$\endgroup\$ – ThreePhaseEel Dec 8 '16 at 4:13

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