Components in circuit help!

Thread Starter


Joined Apr 12, 2012

I am trying to design a circuit that uses an Arduino board to control a solenoid valve that works off mains power. I think I have worked out that I need a resistor between the pin on the board and the base of a transistor, with a diode across a relay and the 5V power from the board into the collector side of the transistor and the emitter of the transistor to the ground. The other side of the relay is just the solenoid valve connected to the power.

I am struggling however to work out which transistor/resistor/relay/diode I need on the board side of the circuit!

Any help would be greatly appreciated, thanks in advance



Joined Sep 9, 2010
You'll need to provide details regarding your relay and solenoid, particularly how much voltage and current they need. This will help determine the other components.


Joined Sep 9, 2010
... I would recommend getting a solid-state relay.
I don't doubt your statement, but just curious...why SSR? I would assume water control is very low duty, and thus a mechanical relay would be fine for a long time. Are SSRs cheaper? Maybe lower control current? Personally, I have a bunch of old relays pulled from TVs or whatever whereas I'd have to go buy an SSR. So I'm biased. ;)


Joined Oct 2, 2009
If you already have a TTL level mechanical relay and your application is very low duty then why spend more money on an SSR?

Also if you are switching an inductive load, a mechanical relay would be better.

When would I use an SSR?
- high switching frequency
- zero mechanical noise
- more reliable


Joined Oct 15, 2009
You need a SPDT or DPDT relay with a "coil voltage" of 5V (or whatever voltage the power source you are using for your arduino is) and make sure the "contact rating" of the relay is sufficient for your 240VAC 1A load. Also get the parts in the pdf I posted above.. I'd just recommend the Tip102 transistor route just in case.
The relay power/gnd in that pdf can just come right from the Vin and GND pins on the arduino.
Then you wire up the solenoid through the C and NO contacts on the relay to turn the solenoid on when you make the arduino pin high or hook it through the C and NC pins if you want the solenoid on when the pin is low..