I am software programmer and hobby farmer. While I was working on a hydroponic farming stuff, I had to use submersible pumps with timers. I don't want to buy timers that are actually a tedious scheduling process. so I am making one myself.

I want to toggle 230 V for turning a submersible pump(150-230 V and 18 W) ON/OFF. I read that Arduino may not able to supply enough input current that the relay requires. I am not sure about what transistor to use to amplify the input current. I came across a circuit diagram and a relay board. Do I still need to use a transistor and diode if I use this relay with an Arduino?

This is the two-channel relay I am about to use and the circuit is below.

Circuit to use relay with Arduino

Should I follow the circuit if I am using this relay? The relay has a tolerance of 250 V. What if the voltage fluctuates above 250 V? I am going to use a 9 V battery to power the Arduino. Feel free to explain anything that I may have to know.


Thanks marla for making it clear that I don't need diodes and transistors. Andy, as you said, I can't keep changing batteries often, and it will limit the system. Shall I use an adapter which reads output 5 V and 1 amperes? Will that drive the Arduino and be sufficient for both the relays?

  • The relay board you will be using already has opto-coupler and transistor driver on the board. You can directly drive the board from your Arduino. Check out this stack exchange page (similar product) : electronics.stackexchange.com/questions/18908/…
    – Marla
    Commented Jun 21, 2014 at 13:21
  • The relay coil will consume maybe 50 to 100 mA. This will reduce battery life dramatically.
    – Andy aka
    Commented Jun 21, 2014 at 13:27
  • @Andyaka could you please take a look at the update ?
    – Sakthivel
    Commented Jun 22, 2014 at 7:21

1 Answer 1


The Arduino will easily drive this relay board, and it needs only a very small input current.

The board you linked to has a built-in optical isolation circuit, so there is no direct electrical connection between the Arduino-side of the circuit and the relay side. This should protect your Arduino if something goes wrong. For what it's worth, the product description specifies this board is suitable for "AC [...] appliances such as [...] motors". A pump is primarily a motor.

The voltage may fluctuate when the pump is started and stopped (Back-EMF). It's hard to say what this might do to the relay board if it goes way over specification. Perhaps you might be happier with a much more robust solid-state relay (SSR) that has a higher rating than the target load. Something like SSR-25DA 25A Solid State Relay - White. Since solid-state relays are non-mechanical, they will not wear-out from repeated switching either.

You may also want to build some fail-safe sensors into your project. For example, water level float switches for both high-water and low-water marks.

  • I am not using solar power now. but thinking about failsafe. i power arduino from wall wart dc supply. so incase of mains failure i want to supply from battery so my arduino wont restart and mess things.
    – Sakthivel
    Commented Jun 26, 2014 at 6:07

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