Simple AC motor On/Off

I want to turn an 240V AC motor on/off with the arduino board.

Which is the best way??....

  1. Optoisolator + Relay
  2. Darlington transistor + relay (do I need protection diodes??)
  3. other... (please suggest)

Please save me from my ignorance!!

Thank you!


Hello Jon!

Welcome back:)

ok... when using ac there's a few options. a mechanical relay (in this case see my reply to your other post) you always need the diode in parallel with the relay, any coil that you switch on and off will send you back some nasty voltages that will eventually kill your transistor.

another option is SSR Solid State Relay it's a relay that is built using only solid state components (mostly TRIACs) very nice because they can be connected to arduino straight away, they have an internal optocoupler and there is no mechanical parts (which also means no spark when switching) they are usually more expensive though.

for example see farnell part number 1213176

If you need to dim an AC light there is a very nice Velleman kit that costs something like 12 pounds that can be used to control an incandescent lamp using the PWM out from an arduino. (it's calleds something like voltage controlled dimmer)

you can also build your own SSR if you want to save money, consider that this usually is trouble because of the EMI (interference) generated by not-properly-designed circuits. for the benefit of global knowledge look at figure 9 at page 6 of this datasheet the schematic shows a simple SSR where Rin is usually 270 ohm... the optocoupler is a special part MOC3021 that's able to fire triacs. there is also a similar part (MOC30something) that fires only when the ac voltage is crossing the 0 which is quite nice for reduced noise.

I hope this covers the situation a bit.

geez I'm so messy when I write :)


Thanks Massimo! SSR is the answer!

And I'll go for the pre made version.


I want to turn on and off a single phase, capacitor start, AC motor. low frequency - on for 10 secs, off for 2 minutes.

  1. Does it matter where I connect the SSR in the AC motor circuit? Can I just put it anywhere on the supply line? or do I need to take the capacitor into consideration??

  2. As far as I understand it, all I need is the arduino board, the SSR, the motor and motor start capacitor. Is it really that easy?? No diodes, resistors or capacitors?? (please say yes)

  3. Excluding voltage and current ratings, is there anything I need to consider when choosing a SSR??

  4. Are all SSRs opto-isolated??

Sorry for all the questions..... Thanks for all the answers!.....