Arduino and Sprinkler System Solenoids

Hi there,

I have been reading up a lot on controlling solenoids from the playground and various websites.

My solenoids that I need to power has the following specification:

• Solenoid: 24 V ac
• Inrush volt-amp: 24 V ac-11.50 VA
• Inrush current: .4 amp
• Holding volt-amp: 24 V ac-5.75 VA
• Holding current: .2 amp

Is it still advisable to make use of an separate 24V AC PSU and use opto-couplers to switch the solenoids? Or would using transistors be a better option?


** bump **

Check out our OpenSprinkler project:

I'm old school when it comes to controlling things like this. I use a small relay hooked to a pin on the arduino, either through a transistor or direct to the pins, then control whatever I want with the small relay. I have a 20A contactor controlled by a tiny relay in one instance and several 10A relays in other projects controlled this way. I usually put the small relay in a socket of some kind so it is easy to change, but haven't had to do that yet. The coil voltage on the big relays ranges from 24VAC in most cases to 110VAC for the big contactor. I also directly control a couple of small 110VAC motors with the little relays.

There are several 5V relays that can hook directly to the pins on the Arduino and don't seem to cause any problems; even after years of use. If you want to be sure you're not overloading the Arduino, a 2N2222 transistor can be placed to buffer the current. Just be sure to put a reverse biased diode across the relay coil to keep back EMF from causing problems.

I did it this way because I got tired of calculating how much current each device could handle and then heat sinking them. A relay is simple, either it's a short or not.

I agree using a relay is simple, but I prefer using triacs because they much smaller in size and significantly cheaper. They are easy to use (connect them just like standard transistors) for switching low-voltage (like 24V) AC power. For high-voltage switching I would prefer using a relay as it decouples the control circuit with the load circuit and is thus safer.

Hi Ray and others,

Take a look at: for a lot about relays...

Relay boards are being manufactured in such large quantities these days it's hard to beat the price, even if you bought the relays and made your own board. And low-cost opto-isolated boards are really good. Some examples:

DISCLAIMER: Mentioned stuff from my own shop...

I'd be happy to try to answer any specific questions about relays ....