I'm very new to EE so forgive the simple question. I have a small water pump 120Vac that I want to turn on and off via my arduino. I have the arduino starter kit as well as a 12v relays.

I've tried to get the relay working and can't even seem to do that. This is the instructable, which I'm trying to do.

I understand that I have a 12v relay, which means I need a way to send more current through to the relay than the arduino 5v can provide. so I tried the smaller transistors (bc5478) in the arduino starter kit, along with one diode that also came with the kit. I thought the transistor had enough juice to activate the relay, but when I put a multimeter to it, it switches between 7 and 9v.

I should also mention I have a 9v battery connected to the relay which should power the pump.

Conceptually, am I doing anything wrong? Maybe I just hooked it up incorrectly?


1 Answer 1


The problem with the relays isn't that of current, but of voltage. You will need to supply 12V to the coil of those relays in order to get it to energize.

One option is to find relays that only require either 5V or 9V instead, so that it can be either powered from the Arduino's supply or from a 9V battery. You will still need the transistor if using 5V relays, since at that point the coil will need more current than the Arduino output driver can provide.

Another option is to draw power from the incoming AC line and put it through a regulator to get the 12V required to operate the relay. Additionally, a 5V regulator will get you the power you need for the Arduino so that the entire apparatus is powered off the line.

  • \$\begingroup\$ thanks. few questions - do you know how much current that relay needs? i couldn't figure it out from the datasheet. also, which AC line are you talking about? lastly, can what I want to do be done with the parts in the starter kit? \$\endgroup\$ Commented Nov 14, 2013 at 4:59
  • \$\begingroup\$ @MatthewBerman: The current will just be the voltage across the coil divided by the resistance of the coil. The AC line I'm talking about is the AC line that the relay is connecting in order to drive the pump. Simply draw from it before the relay so that the power is always on (when connected to the mains, of course). Unfortunately you will need a few parts that aren't in the starter kit. What you can do is disassemble (or integrate) a wall wart in order to get a DC voltage of about 24V and then feed that into a couple of switching regulators for 12V and 5V. \$\endgroup\$ Commented Nov 14, 2013 at 5:27
  • \$\begingroup\$ Ok. I purchased a 5v relay shield so I think that will simplify the whole thing. \$\endgroup\$ Commented Nov 14, 2013 at 5:31
  • \$\begingroup\$ @MatthewBerman: Make sure that the contact rating on the relays on the shield is sufficient to run the pump. \$\endgroup\$ Commented Nov 14, 2013 at 5:32
  • \$\begingroup\$ it says 250vac, right?: amazon.com/gp/product/B0057OC5O8 \$\endgroup\$ Commented Nov 14, 2013 at 6:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.