Control of transistor as switch at power up and power down

Question

I need to replace a physical switch with a transistor. I have a 12V supply which is always on, and i have a 12V control signal.

I need some way of generating a short pulse on both the power up of the 12V control signal and the power down of the control signal. It must be a short pulse (to replicate a short button press <1s >500ms).

I looked at 555 monostable timer circuits but they need to see high-low-high. Where as I can provide only high-low and low-high.

Can anyone provide any hints on possible ICs or circuits to achieve this please?

Do you have a transistor in mind already? Or are you unsure on which to use? — Reinderien, May 17 '18 at 08:09
@Reinderien i hadnt got that far yet, the more pressing concern was how to pulse it. Ill spec the transistor later — Pop24, May 17 '18 at 08:20

score 3 · Accepted Answer · answered May 17 '18 at 08:16

3

Without going into too much detail I would consider the exclusive or gate edge detector circuit: -

This can be scaled up in time by using much bigger values for the capacitor and resistor of course. See also this answer for a rising/falling edge detector - it goes into more detail.

answered May 17 '18 at 08:16

Andy aka

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brilliant, thanks for this! if only i had thought to google edge detector, whicch is exactly what i was looking for. – Pop24 May 17 '18 at 08:20

Control of transistor as switch at power up and power down

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