What components to use to electronically switch a 24v power source

I have an automated watering system which puts out 24volts on a zone to open a valve. What I am trying to achieve is to switch a pressure pump on when a specific zone is switched on by the water controller. To achieve this I need to use an eletronic valve on the water supply output which when open will kick the pressure pump on. To achieve this I thought I would hook a wire into the zone of the controller and use a regulator on the line to reduce the power from 24v to 5v which would go to an ardiuno input pin. When the power is detected on this pin I want to allow power to flow from a power supply through a regulator to open the electronic valve hooked into the water source therefore triggering the pressure pump. When the zone power drops it would then electronically switch off the power to the electronic valve.


Hi Dave
Welcome. There are a couple of guys on the forum that have delved into plant watering.
A few questions.
What flavour is the 24V ?? AC or DC. From memory I think someone mentioned the operating voltage for this sort of thing is 24VAC.
What is the power requirements of the pump.

If you just want to switch on the pump when a particular zone is switched on I don’t think you will require an Arduino.

A block diagram of what you are hoping to achieve would be a big help.
Cheers Bob


From what you have written I think you want to sense when the 24VAC is active.
(assuming it is AC for now).

DO NOT do this.

The best way is to use Opto Isolator. Using the one below, convert the 24VAC to DC, use a resistor divider to lower the voltage and reduce the current, then apply it to an Opto Isolator, the output then connected to the Arduino.

This Opto Isolator provides 5KV of isolation and is designed for DC input, therefore conversion of AC to DC it required. The diode in this Isolator has a max reverse voltage of 5V, applying 24VAC to it would probably kill it. The resistor divider would reduce the voltage and limit the current to less than the 25mA max of the diode.

There are other Opto Isolators that will handle AC input by having two input diodes, then AC to DC conversion would not be needed. In all cases the Absolute Maximum ratings should be noted and reduction of the current and voltage through the diode should be designed.



Hi James

I think you are probably right.

This is probably right too.
The word “probably” is all important here. Maybe wait for more info from David before too many confusing assumptions.

Most definitely agree to that.

Re opto.
This particular one is open collector (NPN) output so Arduino would require pull up or set the input to PULLUP.

Change “probably” to “will”

They will also be quick enough to follow the AC signal and would produce 100Hz DC pulses at the output. Like an opto isolated rectifier which, unless the pulses are required, could be filtered at the output stage.

The 24V to the valves has to be switched by something. Maybe simpler to use this switching instead of dealing with the 24V.

More information from David. I get the feeling this can be handled simply without any Arduino intervention.

Great believer in the KISS principle.
Cheers Bob


Yes I would need to reduce the voltage which comes from the water controller on the zone to 5v to for the arduino to detect when this has been activated by the water controller. Then when the voltage is detected I need to switch on 24 volts power to a solenoid to open and allow water to flow and this would then trigger the pressure pump because the water pressure will drop. If I have a 24v power adapter I believe I need to have a electronic switch between the solenoid and the power source which would be triggered by the arduino. Does this make sense if so what parts do I need on the solenoid side?

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Hi David
You have not answered the questions.
AC or DC
Pump power requirements, 12V, 24V, 240V AC or DC or what ???

Not really. Some sort of diagram of what you are trying to do would work wonders. Just guessing otherwise and guesses are usually wrong leading to a lot of wasted time and research and creates confusion.
Cheers Bob


Hi David,

Can you send us a photo of the plant watering system’s control box? We might be able to work out if the 24V output is AC or DC based on the symbols on the unit.
As Bob and James have pointed out the solution changes entirely based on if the 24V your system outputs is AC or DC.


If AC use something like this.

I have used them successful triggered by a Raspberry Pi Pico, Arduino would work also.

Alternatively you can use a DC relay which would work to switch both AC & DC, but draws more current than the solid relay.

In any case it is hard to advise with limited information on what you want to achieve. Some pics might help, a circuit diagram even if only a rough drawing. In these kind of things I always look for the simplest, easiest, cheapest and most reliable way to do it. Possible there is a better solution in your case. More information would help greatly.


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Hi David
I will have one go at guessing what you want.
Here is a suggested sketch

The 5V water solenoid switching can be from Arduino, switches or whatever. As long as it is 5V.

The relays can be SSR (solid state relays) or opto coupled mechanical relays
You have already stipulated a 24V water solenoid supply but to use commonly available SSR this HAS TO BE AC voltage. Can be DC if “switch at any time” relays uses but “zero crossing” relays more common. The same applies for the pump power supply.

I have only shown 2 water solenoids but this can be as many as required. Just connect the diode as shown then all the diode cathodes together then to the pump relay.

Diodes can be 1N400X types.

In operation the pump relay will operate when one or more of the water solenoid relays are switched.

I have stipulated 5V for the relay switching. This is pretty important as if 3.3V is used the voltage drop across the diode (0.7V) might prevent the pump relay from operating

For your info this arrangement is known as “diode OR” and is quite common practice.
Cheers Bob