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Non-technical person here, so please be gentle. I moved into a place that has ceiling speakers in each room (I tried, but they don't come apart, aren't marked other than manufacturer, and that is a company that went out of business 15+ years ago). So, let's assume they are 8 ohm speakers.

I have a media amplifier, and have successfully played music over all the speakers so I know that they work.

My wife would like the ability to select which speakers are on or off, so I'm thinking a quick Arduino project and relay board with 8-12 relays... but I'm not sure how to estimate the load, so I don't know how to appropriately size the relays.

Any suggestions on the best way to do this, or should I just get mains-rated relays and over-engineer the heck out of this?

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  • Unless you're working with kilo-watt level audio equipment it'll be hard to under engineer it with relays.
    – Majenko
    Commented Mar 15, 2016 at 21:24
  • Bear in mind you may need transistors to drive the relays. Don't forget the snubber diodes over the relay coils. You can buy "speaker switch boxes" which might be a simple solution.
    – Nick Gammon ♦
    Commented Mar 15, 2016 at 21:58
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    Pretty much any "Arduino" relay board on eBay will be more than adequate and contains all the drive circuitry you need.
    – Majenko
    Commented Mar 15, 2016 at 22:31
  • Relay's will do the trick. You might try FET's or transistors, which might become under-engineering at some point.
    – aaa
    Commented Mar 16, 2016 at 12:32
  • You wouldn't even need an Arduino, to be honest. Just put in some switches that power on/off the relays. Unless you would like to make some kind of web-interface or timing schedule or anything that's not 1-on-1 connected with a switch :)
    – aaa
    Commented Mar 16, 2016 at 12:34

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If you can estimate speaker power, it probably is reasonable to use the ordinary power formula P = I²R for estimating the current the relay needs to handle. Typically, high-power speakers of a given kind are bigger than low-power speakers of the same kind. But as you can see from pictures of 5W, 10W, and 40W speakers, some 10W speakers are smaller than some 5W speakers, and some are bigger than 40W speakers, so power estimation based on speaker size can be difficult.

If you have access to the sound source (tuner or player or whatever, connected to an amplifier) you may find output power listed on the amplifier.

In any case, due to the I² term in the power formula, the range of current to plan for is relatively small. For example, if 5W is going to an 8Ω speaker, 5/8 = I², and I = 0.79 A. If it's 40W, then I = 2.24 A. This range is likely to be handled ok by relays sold for ordinary use with Arduino systems.

Note 1: A relay's current rating ordinarily is derated significantly when the relay is used to switch DC or high-voltage AC. Audio is low-voltage AC and relay derating probably isn't needed. Note 2: The current a relay can safely switch in a live circuit may be much less than the unswitched current that closed contacts can carry. This might not be a problem with low-voltage audio. Note 3: Some commercial high-wattage audio distribution systems operate at higher voltages (see, eg, wikipedia's high-voltage audio distribution systems article, re 70.7 V etc systems); if you have one of those – as amplifier labeling might reveal – the example calculations shown above would need adapting.

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