Best way to sense the on/off state of a 24 vac appliance


I need a switch that turns on and off (switching the 3.3 volts on the photon on and off) when an appliance, powered with 24 vac, turns on and off. Something that I don’t have to physically connect to the A/C circuit would be best… perhaps a sensitive hall effect sensor might work. If I connect to the wires then a pcb mount solid state relay would definitely work… but there are so many of the out there!

If anybody in the community has experience with this they might save me a lot of time and headache by sharing! Please be specific. I’m looking for something small, reliable, and inexpensive.

This is one of my first posts… thanks in advance


If you want to connect to the 24Vac lines, you could also use a opto copuler with a resistor, that way you will still have isolated the 2 circuits.


Use a CT perhaps? E.g.:

This won’t transform a 24VAC signal to a 3.3V logic signal - instead you will need to sample this with the ADC to determine if you detect a current. But that is fairly simple to do and the CT is non-invasive i.e., you don’t need to cut your 24VAC wire.

There is a wide range of CTs, so if the one above doesn’t float your boat, look around a bit.


I am onboard with the current transducer suggestion. There are some that generate an analog voltage relative to the current sensed, there are some that close a switch when the current crosses a threshold, and there are digital ones that are probably overkill for most uses.


Both ideas have merit.

I appreciate the info from MORA regarding opto couplers… I was familiar with solid state relays, but opto couplers seem to be more in line with my application. I’ve been researching them online. Apparently I’m looking for an AC optocoupler. Most of the ones I’ve found are similar to relays… a small signal is used to switch a larger signal. My application is a bit different since I’m switching a small dc voltage with a relatively high AC voltage. I’m confident that with some research I could find one ideally suited for my application. This option’s advantages include relatively low cost, and more immunity to RF interference than the competing option. The disadvantage is it requires a physical connection to the circuit.

I’ve also been looking at CTs’. My concerns with the CT are relatively high cost, possible vulnerability to rf interference (relatively high power vhf radio transmitters are nearby). The advantage, of course, is no physical connection is required.

Expanding the idea of “isolation” a bit further… keeping in mind that this option would cost more… I’m wondering if any type of wireless solution is available… clip something on to a wire that signals its on/off state to its mate… Brainstorming… perhaps others have some ideas.

On a side note… I appreciate that the community is responding and contributing to my inquiry… Thanks folks!


@scraphound, how is the 24VAC being supplied - wall wart, transformer, other?


An option you might consider, based upon the amount of current in the 24vac circuit is a “current switch”. We use these all the time to sense when a circuit is functioning. The interface is quite simple – it’s simply a dry contact closure when sufficient current is flowing. An example of a current switch we’ve used would be a Dwyer model CCS-221100. The list price is $20, but if you’re a contractor/OEM there can be a nice discount. A nice feature is the fact they also have an integral LED that shows if the device is detecting current.

I’ve also used simple 24VAC coil relays when interfacing with HVAC equipment. Finally, Fairchild makes a MID400 which is a solid state voltage detector that might work depending upon your application. This is a nice device that we’ve used to detect power failure situations.


@ctmorrison, the CCS might need more current than the “applicance” uses but I really like the MID400! :heart_eyes:


I just looked at the Fairchild MID400… If I’m going to physically hook up to the wires it will do the trick (I’m going to order some and try them out).

The 24 vac is supplied by a wall-wart. I don’t know how much current it draws when on… I’ll have to go out there with a clamp-on meter and find out. The Dwier current switch may or may not work depending on my findings.

I’m also having some luck with a hall-effect switch, but I’m still looking for the magnetic sweet spot to place my sensor on (it may or may not be present).

I appreciate the input from all of you!


The coupler I used last time is a SFH6286, mostly because I needed an AC version with 8mm+ clearance between input and output.

This one is a AC model which really just means it has 2 LEDs inside, so they can shine with voltage applied in any direction, it will still give pulses not a constant signal when applied to AC, so it needs some post processing either in software or hardware.

On the output side, you get a transistor, so generally you use that to pull a signal low/high on your side of the circuit.


Dont forget that a CT is just a coil with a bit of ferrite. You can easily make a coil you put your wall wart psu line through and amolify the voltage to your liking with an opamp.

Just sayin’


You can use an optically isolated digital input board.
like this
it input voltage range is 0-30V AC/DC.


Winding my own coil would cut down on cost and give me more flexibility in design. I’ll might order some ferrite cores and magnet wire and give it a spin.It makes sense to explore all my options up-front. I’d hate to end up a year down the road having made a bad choice.


I checked it out. It’s kind-of pricey but it would do the trick. I’ll keep this on my list of possible solutions. If my potentially lower cost options don’t work out I’ll give this one a try.


I ended up trying the MID400 optoisolator chip… it works! Thanks for the pointers folks!