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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?

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  • \$\begingroup\$ Do you have a transistor in mind already? Or are you unsure on which to use? \$\endgroup\$
    – Reinderien
    May 17, 2018 at 8:09
  • \$\begingroup\$ @Reinderien i hadnt got that far yet, the more pressing concern was how to pulse it. Ill spec the transistor later \$\endgroup\$
    – Pop24
    May 17, 2018 at 8:20

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

enter image description here

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.

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  • \$\begingroup\$ brilliant, thanks for this! if only i had thought to google edge detector, whicch is exactly what i was looking for. \$\endgroup\$
    – Pop24
    May 17, 2018 at 8:20

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