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I have a push button switch that is single-pole, that I need to independently control two circuits. The button will be connected to a digital pin of two different microcontrollers. So its a low current application. Unfortunately, in this case, a two-pole button cannot be used. Is there a way of doing this?

Obviously the first that springs to mind is opto-coupling, however, there is a chance that one circuit is not powered, which would mean there is a chance the opto-coupler is not powered.

It cannot be guaranteed that these two circuits share the same power supplies, hence the complications. Technically the two circuits should be +5V, however the grounds aren't necessarily common.

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  • \$\begingroup\$ The button would enable current flow through the diode of the optocoupler, so there is not problem if it is un powered \$\endgroup\$
    – Claudio Avi Chami
    Apr 28, 2017 at 21:43
  • \$\begingroup\$ You really need to describe your problem if you want a worthwhile answer. Is this a signal (low current, low voltage) or power (higher current, higher voltage) application. For example it could be a 110 V switch for a light circuit ....or it could be a switch input to a microprocessor at 5 V maximum. Describe your two power domains. \$\endgroup\$
    – Jack Creasey
    Apr 28, 2017 at 21:50
  • \$\begingroup\$ Added some more information to the original question @JackCreasey. The circuits are both 5V circuits. \$\endgroup\$
    – CircularRecursion
    Apr 28, 2017 at 21:59
  • \$\begingroup\$ If you don't have either a power supply that is available when either of the two controlled is active or a shared power supply common, you probably can not do what you want to do. \$\endgroup\$
    – user80875
    Apr 28, 2017 at 22:33

3 Answers 3

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How about a DPST Relay? Ideally, you'll want the COILVOLTAGE to be one of the voltages you are using for 2 circuits (VCC1, VCC2).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ What happens when coilvoltage is 0? \$\endgroup\$
    – CircularRecursion
    Apr 28, 2017 at 21:49
  • \$\begingroup\$ Nothing useful. The relay just stays open. \$\endgroup\$
    – Hearth
    Apr 28, 2017 at 21:53
  • \$\begingroup\$ DV for a Page 1 Line 1 error: missing a diode off the relay coil and scorching your switch contacts. \$\endgroup\$
    – TonyM
    Apr 28, 2017 at 22:05
  • \$\begingroup\$ @TonyM. Small switches are rated for 125 V AC and 12 VDC typically .....and they don't need protection diodes across relays. There will be a spark on breaking the circuit, but it won't impact switch life significantly at all. Certainly not worth a DV. \$\endgroup\$
    – Jack Creasey
    Apr 28, 2017 at 22:47
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    \$\begingroup\$ @jackcreasey, the arcing will damage the switch contacts and needlessly. No ratings on these switches here. Are you advocating saving a 2p diode to see if you can get away with it? But above that, the schematic is generic and therefore should always clearly show good design practice. Add a diode, I'll scrub the DV, \$\endgroup\$
    – TonyM
    Apr 28, 2017 at 22:58
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A simple diode isolation will work for you:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ That's how the circuit is currently done, however I cannot guarantee that both the grounds are at 0V. \$\endgroup\$
    – CircularRecursion
    Apr 28, 2017 at 22:48
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    \$\begingroup\$ The 0 V level for each system would be connected together by the switch. It does not have to be a true ground terminal. I don't see a way to connect a switch to two system without a zero V reference in each system. For example you could have two battery powered Arduinos ...completely floating from a real ground, but you could connect as above and have the switch work. \$\endgroup\$
    – Jack Creasey
    Apr 29, 2017 at 2:24
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Hmm.. tricky set of conditions...

You can do it with transformers and a function generator to make a fake isolated supply to drive the led of an opto-coupler.

Though if you are willing to detect a modulated signal back you could probably get rid of all that regulator stuff.

schematic

simulate this circuit – Schematic created using CircuitLab

Though I am pretty sure there is a way to do this from the primary side, but I cant seem to simulate it.

Or, you can use one of these

enter image description here

schematic

simulate this circuit

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