Relay questions

Quick/silly question about relays:

Can you use a relay with a 5v coil to switch a 9v or 12v signal? say you wanted to use lower volt arduino logic voltage to power the relay coils for switches carrying synth level signals

Or is it better to use your low voltage on a relay driver transistor that opens up the bigger voltage to the coil

Data sheets show minimum trigger voltage and “nominal voltage”

Bit confused


That’s the whole point of relays…
Low voltage to drive higher voltage.
You should use nominal voltage for the coil…


The coil and the switch(es) are two different things. You can switch a 240 V AC signal with a 5 V coil, if the relay is built for that.

Here are the specs for the first relay google showed me; first image is the switch specs, second the coil variants (this one is available for 5−48 V)


Awesome yep yep I figured I was missing something. This will be my first project using relays. I have a random 12v relay here I was messing with.

Could get it to click with voltages down to like 8v with my bench supply, but trying with a 10v gate signal from my synth wouldn’t do it…weird, so got my meter out, connecting the coil droped the voltage to like 7v and so no click.

Ultimately I just need 5v gates to trigger a signal potentially +/-12v so any ol 5v mini relay will probably be great

I’m looking at these really cute ones that fit in the footprint of a dip-16


connecting the coil droped the voltage to like 7v and so no click.

Yeah, 28 ohm (or whatever your relay has, probably a bit higher if you got 8 V) is pretty low for a normal synth output stage. Here’s the output you can expect from a TL07x powered by +15 V:

(28 ohm would be to the left of 0.1 in that graph)

Adding a transistor on the way out would solve this (as long as it can handle the current, that is).

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Just be sure the coil resistance/impedance is high enough.
That’s why your gate dropped to 7V.
With the usual 1K output impedance of modules, your relay’s coil must be quite higher to avoid that drop, because that form a voltage divider.


This might be useful:


This is all useful thanks everyone

do yo realy need a relay, would a solid state switch do the job

something like a CD4066

No I definitely don’t need to use a relay. But I wanna. Lol.

It’s for an old RadioShack mixer I’m working on fixing up and modding.

Specifically adding mute and solo switches. Mute is easy, solo is a little trickier. Idea is Solo switch on a track opens the relays on the other tracks, as long as they don’t also have solo selected “on.” With indicator LEDs to show if the tracks are on. Using dpdt relays with dpdt toggle switches u can kill two birds with one stone there. (4 birds with 2 stones? :rofl:)

Hopefully I can also add gate inputs (hence my noodling around with my only relay). with switches to control if you want the gate to activate either the mute or the solo

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Mosfets would probably be the “best” solution

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Here’s my idea so far. The main mixer pcb will be original so it’s not shown. Got a few different relays ordered. I’ll have to test the transistor driver/relay combos with my gates to be sure

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Just breadboarded this with a 3904, 1k on the input, and 10k to ground. Relay coil connected to 12v.

Works like a charm! Pretty cool to hear the relay turn over at audible frequencies :rofl:


Program it to play this

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I’m going to need some more relays :rofl:

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:rofl::rofl: might have to add piezo outputs to my mixer I’m fixing


(not a fan of articles that imply that you can or should design for an exact hFE (“it needs to be perfectly calculated” no it doesn’t at least not in the way you’re doing it there) but CTorp’s schematics should work just fine, at least as long as the relay coils don’t need a lot of current.)


I don’t see a minimum current for the coil?

I see a switching load.

That’s the switch, but on the driver side it’s the coil that’s important. Is that Fujitsu’s RY Series or something else? If the former, there’s a whole bunch of coil rating tables later in the datasheet (and enough variants that you either have to find the right table for us or post the exact partnumber).

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Yeah, RY-9-WK

150mW at 9v/540r = 16.6 mA of I?