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Electrical switch

Lets say that I have a microcontroller (MC-A) with a digital input that is actuated by a switch.  I have another microcontroller (MC-B) which I want to use to trigger the switch (as if the user pressed it).  I assume that I cannot simply attach the digital out pin on MC-B to the digital input on MC-A.  So I would need a relay or something which would allow MC-B to bypass the switch.  Relays are slow, mechanical devices; my best guess is an optocoupler.

What kind of hardware switch would I need to do this?  It needs to wait for the output pin on MC-B to go HIGH, and then allow current to flow and bypass the push button switch.

MC-A is running at 3 volts, and MC-B is running at 5 volts.

I am a complete newbie at this.
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ChetOS82
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ChetOS82
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ComputerTechieCommented:
check this out

http://www.newark.com/pdfs/techarticles/microchip/3_3vto5vAnalogTipsnTricksBrchr .pdf

I have not done this myself.

CT
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ChetOS82Author Commented:
Sorry, I am a newbie; I am not qualified to evaluate these methods :)
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CallandorCommented:
>What kind of hardware switch would I need to do this?

I would say a solid state relay with the voltages and current you need.  There are no moving parts.
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ChetOS82Author Commented:
The solid state relays are rated for hundreds of volts, which makes me think they are not designed for low power use (although they will probably work).

Could I not just use a voltage divider on the input (to go from +5V to +3.3V), and connect the grounds of the two controllers?
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CallandorCommented:
If you're connecting the grounds, how are you going to make it function as a switch?  That configuration sounds like it's always on.
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ChetOS82Author Commented:
+5 is coming out of a digital output on MC-B.  Can I connect it to a digital input on MC-A (which is a 3.3V chip) if I use a voltage divider and connect the grounds?
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CallandorCommented:
The +5v output is controlled by MC-B, so it acts as a switch?  As long as the current draw of MC-A on that pin does not exceed what MC-B can provide, you can do this.
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ChetOS82Author Commented:
That is what I needed to know.
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