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I have a basic doorbell that I would like to control using my Arduino. The doorbell runs on 220V AC power and is ordinarily controlled through a secondary circuit(??) running 7.5V AC.

My question is - since the control is 7.5V AC, what is the correct way to interface that with my Arduino, which typically outputs 5V DC?

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    \$\begingroup\$ You're going to want to know how much current the doorbell requires at 7.5VAC. If you have a multimeter, you can use the current measurement setting and connect the probes on either side of the doorbell button. This will make it so that the meter completes the circuit, telling you the current. This will help you size the right SSR (solid state relay) or mechanical relay. \$\endgroup\$ – HikeOnPast Jan 11 '13 at 22:48
  • \$\begingroup\$ Depending on the type of doorbell it might not be that easy to measure, it might be safer aiming for something that can switch the full rated power of the transformer if it's marked. Or considering it's a one-off project use something like 5A relay which will be more than the likely 1-2A a doorbell transformer will provide. \$\endgroup\$ – PeterJ Jan 12 '13 at 0:26
  • \$\begingroup\$ Thank you both! The sticker on the doorbell also states "secondary current" 0.6A. I guess that's what my relay should be capable of providing? \$\endgroup\$ – urig Jan 12 '13 at 10:31
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    \$\begingroup\$ I'd go for a 1A relay in that case (just next standard rating above). I assume you won't be switching it on and off hundreds of times a day so personally I'd just use a mechanical relay so it's easier to test and you don't have to worry about it working differently with a low-voltage AC load. \$\endgroup\$ – PeterJ Jan 12 '13 at 10:54
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A solid state relay that supports AC load voltage would probably be a good thing to try.

If you can't find a solid state relay that will support that low of an AC voltage you might use a mechanical relay and an optocoupler.

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  • \$\begingroup\$ At his voltage, pretty much any relay would work, as long as the current isn't an issue. He first has to figure out what current is needed. \$\endgroup\$ – Gustavo Litovsky Jan 11 '13 at 23:11
  • \$\begingroup\$ Thanks for the answer! So relays either support AC or DC loads but not both? I have this available right now - canton-electronics.com/…. Am i right to assume I need something else? \$\endgroup\$ – urig Jan 12 '13 at 10:36
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    \$\begingroup\$ That relay would work fine, for mechanical relays they all switch AC or DC the only reason for the different ratings is to do with peak voltage that won't apply in your case. That one has a 12V coil voltage though, if you have that available anyway go for it otherwise you can also find relays that have a 5V coil voltage, although you'll still likely need the transistor to supply enough current. \$\endgroup\$ – PeterJ Jan 12 '13 at 11:03
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Use a reed relay like this one. It will handle your 1A current requirement and can be driven directly by an Arduino output pin, given that it only requires 10mA. In other words, you don't need an external transistor, though you will need a flyback diode across the relay coil terminals to suppress inductive transients.

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I bought a relay shield by DFRobot for this purpose. Now I am going to try an solid state relay, have a look at my blog about the Arduino doorbell.

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  • \$\begingroup\$ Nice project :) \$\endgroup\$ – urig Feb 11 '13 at 14:28

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