Voltage drop over relays (or other solution?)

Hey people!

I'm busy with modifying one of my previously made boards, and came across one question.

Right now this part of the circuit does a simple thing: SPDT switch changes channels of output. Signal (0-5V).
--Above example in the drawing--

What I would like to do is: to keep this functionality, but add a possibility to send a simple on/off state to Arduino, so Arduino would know what channel is running at the moment.

Of course would be easy to use DPDT switch instead, but thats not an option because all SPDT switches are already soldered on PCB. So I can only route those switches via relays (I have no other option in my mind right now). It's a modular thing, all switches are on one PCB and the rest of the circuit on the other PCB, I can basically manipulate anything that is between those.

And my question: is there any voltage drop via relay? Because the voltage of the signal is critical. It is musical stuff and basically it controls notes on the other end. 1V per Octave. It means that notes are changing every ~0.084V

Any other option?


Usually there is no measurable difference, but if you use a Solid state relay, it can be up to 2V

Hi Aukuras,

On the face of it I think it would probably just come down to Ohms law; if there's very low resistance / impedance and/or very little current then I would expect very little voltage drop.

But I really don't know what kinds of currents will be carried by your circuits. Would expect VERY low resistance across most mechanical relays though.

Interesting point about ohms law, My 5v mechanical relays show 0.0 Ohms with a fluke multimeter, so voltage drop shouldn't be an issue with a good power supply. But using a mechanical relay you need to be aware that there is a short moment of poor contact, which can send voltage spikes though your circuit if not isolated. Might be the reason you don't often see this setup in audio.

Can't you just connect an Arduino pin to the top end of the relay coil? It will read 0 when the switch is open and Vdd when it is closed. Or am I missing something?

Have you considered using the currently unused pin of your SPDT switch? Have the Arduino sense when the relay is NOT engaged.

Thank you for all of the replies!

I'm not sure what is low or high current in this situation, but it would be about 5 - 6mA

Actually I knew about that poor contact while switching, but I forgot about it. Good that you pointed that out! Mostly it would not be a problem because switching is made before any sound is played. But I have to think whether could be a situation when switching is made during playout. Hm...

You are absolutely right!

I didn't considered that yet, because I didn't go in the details for the other part of the circuit.
And you are right, there is no need of the dual action relay. Thanks!
First I would like to find out if a relay is at all a good option for me, and I will take Arduino side later on.

You might find this interesting. CD405xB
Might require significant update to your circuit, but low cost, make before break switching, works with signals centered at 0V (dual supply) or single supply operation.

Relay contact resistance will be quoted in its datasheet - a small number of milliohms is normal.

DC and some AC SSR's are MOSFET based and similarly in the low-milliohms range as MOSFETs are resistive.

Some AC SSR's using triacs will have a on-voltage loss of 2V or so as they are not MOSFET

These components certainly exist but I think in the 5v world the breakdown voltage is too low, or the gate signal is rather high. Do you have an example of a SSR that would be appropriate for this project? I think an AC SSR is a triac but maybe there are exceptions.

The G3VM-41GR5 has Ron of only typical, more than 71K available at DigiKey.

Sweet fet! If it wasn't for the price i'd be stocking up on those, thanks for the tip. It reminds me of an optoisolator.

It is a type of opto-isolator, yes... Sometimes called photo FET optocouplers, in analogy with photodiode or phototransistor optocouplers. They are slow, note, and should behave close to resistive when on. The way they work is cunning, there's a series photodiode array lit by the emitter LED, and this generates enough voltage to turn on the FET(s) - like a tiny PV panel inside the thing.

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