Electrical – power on power off pulses to two seperate controllers


I would like to request a circuit where a 500 ms pulse is sent to one rail when 12 volt power is supplied and another 500 ms pulse is sent to a different rail when power is turned off. 555 is the way to go but struggling with sending pulses to two different controllers.

So for power on pulse, I guess I do not need a 555. Here is what I have for this:
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Also, i intend to use a transistor at output so the memory recall will have a short during the pulse, just like button press. Should give me a half second pulse.

Searching for power-off pulse now. I guess a cap that holds some charge, enough to generate a pulse after power is cut down.

Best Answer

You need to generate a signal that activates for 500ms when power goes off, and something to keep your circuit powered during this time. These two functions could be combined in a circuit that looks something like this:-


simulate this circuit – Schematic created using CircuitLab

When power is ON Q1 is turned on through R1, keeping Q2 off. When power is turned OFF C1 holds the voltage up (D1 preventing it from discharging into the power supply) while Q1 turns off allowing Q2 to turn on through R4. C1 then discharges slowly through R4 until Q2 turns off again.

R2 and R5 make Q1 and Q2 turn on/off at higher voltage for better defined switching points. R3 slows down the voltage rise on C1 when power applied to make sure Q1 turns on before Q2 is able to.

To avoid unwanted and possibly damaging ground loop currents you should connect your circuit's ground to the seat controller ground only, which hopefully is on one side of the button. If the button is not grounded then you may have to use some other method of shorting across it (eg. analog switch or opto-coupler).