I'm looking to use this relay with my arduino. It has an optocoupler and correct me if I'm wrong but this should prevent any back EMF from affecting my arduino. Do I still need to use a transistor?
closed as off-topic by jfpoilpret, dhimaspw, Mattia, Paul, Nick Gammon♦ Jan 29 '17 at 6:35
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That isn't just a relay you are linking to. That's an entire board that, among others, contains a relay (blue box). This board already has a transistor to drive the coil inside the relay. It also has a diode to handle back EMF.
It even has a opto-coupler, thought that isn't that useful as in most cases both sides of the opto-coupler are powered by the same 5V coming from the arduino.
Gerben is right. That is a complete board, not a relay. I have used some of these and they do not require a transistor to drive them (although you could use one if power output from the ports are a concern). I beg to differ from Gerben, as the optocoupler isolates the pin from the driving circuit (even if it does not isolate the regulator) it still retains much of its safety characteristics.