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I like to turn on or off an equipment (Humidifier) by just supplying or cutting power to using an external relay.

This equipment has a push button (tac switch) to turn on or off, therefore when I supply power it doesn’t turn on, it needs a button press. Somehow, I need to mimic that action (shorting two terminals, when power become available) using a circuit.

When power is available, the circuit should turn on, wait a bit, short terminals and wait a bit more and open. (It seems a rising edge is needed)

My supply is 12V. How can I do this ? Looking for a circuit which i can implements with very few components so that I can build this and insert on top of the board.

TGG
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  • you are asking about bypassing switch on some unknown equipment ... don't you think that you should say what that equipment is? – jsotola Jun 06 '20 at 01:16
  • It is an Humidifier – TGG Jun 06 '20 at 02:23
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    @jsotola actually what the equipment is has nothing to do with question. It could been a rocket and the approach doesn’t change. The question is how to mimic tac switch behavior. – TGG Jun 06 '20 at 03:11
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    it was not clear of high voltage switching is involved ... for the humidifier, the control panel is probably probably involves low voltage switches ... but the humidifier may possibly have line voltage on its internal components ... you need to be very careful when working inside the device ... even though a switch may have 5 Vdc across it, it could be floating on top of 240 Vac – jsotola Jun 06 '20 at 05:32
  • Could you post the electrical schematic of the humidifier? – vu2nan Oct 08 '20 at 03:07

2 Answers2

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There is a little trick I used for that kind of thing. I used a property of a capacitor: imagine your cap is sitting idle, both sides at 0V. If you apply 5V to one side, the other side of it becomes 5V too.

This is worth trying at least, how about combining that with the capacitor+comparator for initial delay? Generic LM393 would work, I believe. Try it out. Make sure all components can withstand the voltage, you know the stuff.

Use RC constant to find the fitting pause. For me 10k+10uF worked like charm, but I didn't have the comparator part.

So look, made it up for you, see if it works, or maybe you can modify it to your needs: enter image description here

Ilya
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You can in some devices cheat and fit a decently sized capacitor e.g. 1uF across the switch contacts, when the device switches on, it will take some time to charge up, meaning the device sees it as a button press.

Reroute
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