I want a few 230V devices (my entire audio system) to turn on when either one of two switches is activated.

1 would be my raspberry Pi sending a signal (when music is playing)

2 would be me pressing the foot switch to which my TV is connected

I assume this is best realised with two relays. The foot switch has a blue indicator LED with 2.4V across it (when switched on), so I thought to just hook up a 3V relay in parallel to that switch. And a 5V relay for the signal from the PI GPIO header.

This is the schematic I had in mind: schematic

Are there any issues with having 2 relays controlling 1 circuit? Let's say Relay1 is activated, energising the circuit via the NO terminal.

That means that there will be a backfeed to the NO terminal of Relay2. Is this an issue? And if not, would it be an issue when someone hits the food switch and also activates Relay2? I have cheap standard relays if that matters (Tongling JQC-3FF-S-Z for example).

I assume not, but wanted to check anyway if I need any additional components. Or maybe there is an even smarter way to solve my issue :)

  • 1
    \$\begingroup\$ No backfeed issues. The Pi probably can't provide enough current through its pins to drive the relay. You would need transistors to act as intermediary. You may be able to just use one relay and wire the foot switch and PI to the same control coil with some thought depending on how things are (you said something about your TV?). \$\endgroup\$
    – DKNguyen
    Feb 14, 2023 at 16:45
  • \$\begingroup\$ Ah I didn't consider the PI side being a potential issue. Thank you. The foot switch is stand alone, it has a 2.5V circuit for the indicator LED. Thats where I thought I would grab the trigger current from. I think 5V from the PI would kill the LED / I don't want the LED to light up when the foot switch isnt on. Or does the trigger connector of the relay not need any power / voltage going through it? Is it simply about the trigger circuit being open / closed? \$\endgroup\$
    – Simon
    Feb 14, 2023 at 16:54
  • \$\begingroup\$ Well, you can just use two relays if you don't know for sure since less thinking that way. But off the top of my head you get a relay whose coil can operate off 3V and you use a diode to prevent 5V PI power from backflowing through the foot switch, and you use a resistor in the 5V path so you can run a 3V coil off 5V. That said, the diode introduces a voltage drop and finding a relay with a coil that runs off 3V minus that voltage drop could be tricky. \$\endgroup\$
    – DKNguyen
    Feb 14, 2023 at 17:26
  • \$\begingroup\$ Parallel connection gives logical OR, in serial: logical AND. Do not forget flyback diodes. \$\endgroup\$
    – user263983
    Feb 14, 2023 at 18:07
  • \$\begingroup\$ I am not familiar with flyback diodes, can you elaborate which one I should place where please? \$\endgroup\$
    – Simon
    Feb 14, 2023 at 19:19

1 Answer 1


No issues expected. This should work fine, unless you have included other elements which are not shown here.

  • \$\begingroup\$ Thank you! No other elements in my circuit. \$\endgroup\$
    – Simon
    Feb 14, 2023 at 19:20

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