I have two separate circuits with different types of diodes. One circuit draws 3,5V and the other draws 9V. How can I wire this up so that one switch can turn both circuits on or off? As far as I understand I can do this with a relay. What kind of relay would I use?

This is NOT a duplicate of former question

Controlling Two Circuits with One Switch

...as some of the people here seem to believe.

  • 2
    \$\begingroup\$ Please leave a comment if you downvote. Otherwise it's impossible to improve the quality of the questions. Of course I had looked at similar questions and googled the issue before I asked. And no, it's not answered before in the question "Controlling Two Circuits with One Switch". \$\endgroup\$
    – Pedery
    Nov 9, 2018 at 15:15

3 Answers 3


Use a dual-pole switch. DPDT or DPST.

That’s two separate switches that share mechanics.

The same naming convention also applies to relays.

  • \$\begingroup\$ I Googled this extensively (and saw several videos on YouTube) before I posted the question, and didn't get the feeling this would solve my issue. Keep in mind that the two circuits are completely different, have different voltages, different components and different resistance. By flipping a single switch, I'd like to turn both separate circuits on. \$\endgroup\$
    – Pedery
    Nov 9, 2018 at 15:19
  • \$\begingroup\$ @Pedery there is absolutely no electrical connection between the poles of a DPxT devise. Be it a switch or a relay. As long as you don’t exceed the ratings the circuits will be functionally isolated from each other. \$\endgroup\$ Nov 9, 2018 at 17:04
  • \$\begingroup\$ Thanks, then this is exactly what I need. I'll mark your answer as the accepted answer. I assume such a switch can also work in reverse, so that flipping one switch on means the other automatically goes off? \$\endgroup\$
    – Pedery
    Nov 12, 2018 at 17:22
  • \$\begingroup\$ @Pedery for that you would need a DPDT. So you can connect each “throw” differently. \$\endgroup\$ Nov 12, 2018 at 17:46
  • 1
    \$\begingroup\$ All you need is one DPDT relay with a coil that can be directly actuated with the voltage of your primary switch. \$\endgroup\$ Nov 14, 2018 at 11:07


simulate this circuit – Schematic created using CircuitLab

Figure 1. An example of using a DPST switch to control two circuits simultaneously while maintaining isolation.


If they have common ground, put switch on the ground side

  • \$\begingroup\$ "I have two separate circuits ..." \$\endgroup\$
    – Transistor
    Nov 9, 2018 at 15:32
  • \$\begingroup\$ Then it's DPST as mentioned in other responses. \$\endgroup\$
    – stiebrs
    Nov 10, 2018 at 13:53

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