Watering system 2 x 8 Channel Solid State Relay Module

I am also trying to build a Pico W watering system with 16 zones after being inspired by Jim’s project.

The software is pretty much done, and the Pico W seems fine as a webserver, scheduler, and controlling the relay boards.

I am still at breadboard stage, but I am worried I might be using the wrong type of relay boards or just don’t understand how they work.

I can’t detect any AC output voltage from the channel terminals, whether they are open/closed (red LED on/off). BTW: Does the red LED mean the channel is open or closed?

I am using a 24vac supply, 2 x 8 Channel Solid State Relay Module (DC Control, AC Loads), Pico pins delivering 3.2v dc to relay control pins.

I came across this post today which gave me a sinking feeling, but now I am confused because I thought Jim was using the smaller channel versions.


Any advice or links to ‘how-to’ documentation or videos for these types of boards would be appreciated.



I looked at the datasheets and couldnt work it out, however, usually, relays only provide a continuity, not output voltage.
When switched is there continuity across the pins that is not there when not switched?

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I am probably using the wrong terminology… if I measure the AC voltage on the board across the supply terminal I get 27v, I was expecting if I check the voltage across a channel terminal in the same way to get 27v (assuming it’s switched). I am not sure where to test the continuity between, so far I haven’t made it beep, other than the two GNDs.


Check for continuity across the relay terminals.

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On the 4 Channel and 1 channel modules RED means active.
Note: the 1 channel module I had required a low digital to activate, the 4 channel module a high to activate. The web page says low to activate for both. Possibly the modules have changed since I bought them.

The wiring:
One side of the 24 VAC to one side of all solenoids.
The other side to the one of the output pins of the relay.
The other relay pin to the other side of the solenoid.

Thus when the relay activates if connects the AC through the solenoid.

The OMRON G3MB Solid State Relay uses a Triac. It is very different from a coil contact relay. You cannot measure continuity. The AC signal passing through zero is what switches the Triac Off, otherwise it would stay on. When activated the Triac stays switched on because the trigger is always applied regardless of the AC level. When the trigger is removed the next time the AC passes through zero the Triac switches off.

In testing I had a 24 VAC water system solenoid connected and easy to see when it activates due to the click it made.




Thanks Jim. Based on your diagram, I think I have it wired wrong. I had both ends of my test solenoid going into the two spots on the channel terminal. I need to run to kids sports. Look forward to testing it tonight.

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The rated capacity means you can use up to that voltage level.
The modules I have work perfectly at 24 VAC, the 8 channel uses the same ORON G3MB relay.
It was a concern of mine initially, but proved to be ok.

I think if you do NOT have a load connected then the Triac has nothing to drive into and wont turn on.
So you may not measure anything. I had the solenoid from another project and it provides enough load to activate the Relay. (500mA)


EDIT: Measurements.
Relay deactivated. Solenoid connected 0v, NO solenoid 28 VAC.
Relay activated. Solenoid connected and ON 26 VAC, NO solenoid 28 VAC.
So with no load it is hard to tell if the relay is working or not.


Hi Andrew, Tim

As James says. You can’t do that, neither can you switch DC with this device. It only closes and opens at zero voltage crossing of an AC supply. DC (or your multimeter on continuity setting) will not get to zero so the relay will never switch.

You can however get SS relays which switch AT ANY TIME so will switch DC, not all that common though.
Cheers Bob
PS: By the way OMRON have been in the relay business for a LOOOOOOONG time and in my experience have been very reliable. In the past I have used literally thousands of the LY series I think without a single failure (except for maybe old age). With the LY series however they tend to work loose in the sockets (if used) and I have found the retaining clip (mostly optional) to be essential.


Hi James

This would be normal. You would measure ACV no matter what state the relay was in.
This will happen due to a snubber circuit of a capacitor and resistor in series across the relay “contacts”. With the extremely light load presented by a DMM the full voltage would be displayed.

This has another effect. If you are attempting to switch very light loads of only a few milliamps at say 240VAC you would find (as I did) that the switched device would not turn off. The fix here was to fit a mains rated capacitor across the device to increase the load and reduce the voltage available via the snubber circuit to a level where the switched device turned off.

I hope this observation explains potential hair pulling situations.
Cheers Bob
PS: This snubber circuit is built in and there is nothing you can do about it. Just a quirk of these devices.


Based on how I interpreted Jim’s wiring note, I am having some success, but not confident I have it right. My board also works as Jim described the 4 channel, High Pin to activate (opposite to product page).

When the relay channel is activated (red LED on), my test solenoid sounds more like a pump, noisy and vibrates. Is it not getting enough voltage? I tried to get a readying, 16vac.

I have an image of the current wiring.

Is there a trick to uploading an image with a post? It’s like I don’t have permission. I can see the image in the preview, but just can’t Reply.


Hi Tim

That is pretty clear and simple. What don’t you understand about it?

What voltage are you starting off with. If it is 24V that would imply your power supply is not able to supply enough current. You should not measure more than a couple of volts below the no load figure.

Is your test solenoid an AC type. That could be the result if you have a DC device.

James may be able to give you some idea of the expected current requirement or it could be marked on the solenoid. Or oh horror, you may have to look up this detail from the data sheet if you can ascertain the solenoid part or model number.
Cheers Bob


Thanks Bob.

I am using a 24CVAC 1A wall adaptor. Maybe 1A is cutting it too fine? On the GND VCC terminal it reads 27vac.

My test solenoid is a Toro Mini Control valve 24 volt AC, 50 Hz, inrush current 0.5 amp

I agree Jim’s diagram was very clear and helpful, and confirmed I was on the wrong track.

I guess I didn’t understand what’s happening on the board, and how the GND and VCC terminal relates to the relay channel terminals. I was worried about frying something.

On the 8-channel relay board, I have the AC adaptor connected to GND and VCC (that’s the only difference from Jim’s diagram), then as per his diagram, I am looping one side of the AC to the left CH1, right CH1 to the solenoid, other side of solenoid back to ground/the other AC. It works (other than my noisy solenoid not fully latching), it just feels like I am by passing something I incorrectly thought the board might do… I’ve never used a relay board before.

I am going unplug and test another solenoid to see if I can get that to latch.

This is my biggest problem. Starting with 24V (reading 27V across GND/VCC terminal). When I measured across the solenoid while on, I only get 16V.



Small update to my post above:

Test 1
I added a second solenoid to the relay module. Same issue, not getting enough voltage(?) to stay latched. Noisy vibrations.

Test 2
I tried attaching the solenoid directly to the 24VAC adaptor, it stays latched and only makes a gentle hum. There was only a 1V difference when the solenoid is connected, which is what Bob said to expect if things are operating normally.

It looks like I am losing 10V somehow, with they way I am trying to use relay module.


Hi Tim

This is your problem. You are applying AC to the opto coupler and it is trying to switch at a 50 Hz rate.
Please read the description carefully. VCC is logic (5VDC) supply for the opto coupler and switching.
This seems to be that you take the controlling signal to ground (LOW) to activate although I think someone mentioned that they supply a HIGH to activate. Core seem to have hidden or not made available a circuit so I am unable to confirm. You are using that Core product you linked aren’t you. Different brands are likely to behave differently.

As I understand it you connect VCC and ground to 5VDC. As there are only a few milliamps involved this could be supplied from Arduino or similar.
The control signal of each relay is taken to Ground (LOW) to activate.
Consider each side of 24VAC as Active and Neutral as you would in mains wiring.
Connect Neutral to common of all solenoids.
Loop Active through one side of relays.
Connect the other side of each relay to each solenoid.

If you CAREFULLY read the description and follow James’ diagram you should not go wrong.
The only danger here is you may have destroyed the Opto Couplers on the board with the application of 24VAC in which case you will have to replace it. You apparently have a second one to try if you haven’t damaged this one too. Clearly mark the suspect one so you don’t accidentally get it back into the system.

I would be happier if Core could confirm my understanding of the operation as I have outlined above or publish a circuit so can have a look for myself.
Cheers Bob
PS: The 1A wall adaptor will be nowhere near enough to drive more than 1 or at a pinch 2 solenoids. You always have to consider worst case. In this case 16 solenoids all making at once will be 8A. As this will probably never happen you could get away with maybe 5A but I personally would not go lower.


Thank you so much Bob! You were 100% right. I removed the AC supply from the GND/VCC (oh boy, how dumb am I). Added 3.3v from the Pico to the VCC and it’s all working. The LEDs are a bit dimmer :slight_smile:

Yes both boards and PICO are from Core. And they are all still working!

I’ll look for an adaptor with more Amps, but all the code/logic ensures only one zone can run at a time due to water pressure.

Thanks again.



Hi Tim
Good outcome. Tip: Mark that board as suspect. Most electronic damage is cumulative and a problem may not show up for some time. Chances are that if none of the control inputs were grounded at any time while the 24VAC was in the wrong place everything might be OK. Fingers crossed. Call that problem solved.

I would still try to find a 24VAC supply with a bit more capability. I personally tend to go for double what I think I will need and sometimes more. Very rarely have any supply problems that way.
Cheers Bob


I see it is solved in my absence. Been busy today. Thanks @Robert93820 for your help.

Plug pack I use can supply 2A @ 24 VAC.
Measures 28 VAC no load. 26 VAC when solenoid on.
Current is about 0.5A when solenoid active.



Thanks guys, great tips and support. The product page for this one was a little light on for my level of knowledge and I had stuck in my head it must work like a motor controller with the separation of logic and a higher voltage source.

Embarrassingly simple once the penny dropped. Problem solved.



good move trying the solenoid directly on the AC adaptor. An old fault finding tool is ‘divide and conquer’, so trying the relay directly is one of the first tests.
On a slightly different note, have you checked the specification sheet for the SSRs?
They have an Output ON voltage drop. That means the voltage on the relay will be about 1.6 Vac less than your supply (about 6%)so when you come to practicalities of longer wiring runs that may cause issues.

I also see that they quote a Load Voltage range of 75 to 264 Vac, so they will not ‘guarantee’ operation with a 24Vac load.