Need to switch 24VAC on/off

Hi, another noob question here.

I want to switch a 24VAC source on and off (ultimately driving an AC stepper motor) with my Arduino UNO. What's the proper way to do this? My limited knowledge suggests 3 potential ways.

  1. Use a transistor. But can a transistor be switched with VDC yet driving VAC? My reading suggests the answer is 'no'. If yes, any concerns with 5VDC and 24VAC using one of the transistors in the Arduino starter kit? (IRF520 & BC 547B) [I think the IRF520 could work, as I believe it is NPN and since we are talking different voltages, this matters.]

  2. Which leads me to an optocoupler. My starter kit shipped with the 4N35. But here I have the same question -- is it okay to put VAC (and 24VAC at that) on the switched side?

  3. I guess the obvious choice is a relay which can be controlled by the Arduino. I don't have one on hand, and reading things suggests that the Arduino doesn't have the necessary current to close a relay. So I'd need to better understand how to deal with that, if I go this route. Pointers to beginner tutorials are welcome, if this is my only viable option.

Many thanks!!!!

What sort of current are you after?

The way to switch AC is to use a Triac, none of your suggestions will work.

qodfathr:
I want to switch a 24VAC source on and off (ultimately driving an AC stepper motor)

Perhaps you mean a synchronous AC motor? Steppers are multi-phase.

Let me be a bit more clear; I appreciate your patience as I attempt to explain in layman's terms.

I have an electronic damper. It has three terminals on it -- Open, Close and Ground. When 24VAC is applied to Open, it slow opens (45 seconds). It automatically stops at the correct position, even if the 24VAC is continuously applied (and the motor stops trying to go any further). Similarly, applying 24VAC to Close closes the damper. If neither Open nor Close have voltage, it does nothing. (e.g., it doesn't seem to slam shut, even if it was halfway open).

I'd like to control this damper with my Arduino UNO. I though I could use two relays, one driving Open and the other driving Close. I may have a need to drive another 1-2 things in a similar fashion, so I'm looking at this 4 relay board: http://www.amazon.com/gp/product/B00E0NSORY?psc=1&redirect=true&ref_=ox_sc_act_title_1&smid=ADHH624DX2Q66.

Am I making a terrible mistake? Have I over simplified things?

reading things suggests that the Arduino doesn't have the necessary current to close a relay.

It doesn't but that relay board has a transistor on it to drive the relay with the small current that the Arduino can deliver.

Yes a relay is your best bet now you have more fully explained your project.

Thanks, Grumpy_Mike! I appreciate your expertise!