Using 24VAC as an interrupt?

by jarruda on Sat Nov 24, 2012 9:29 pm

Hello all!

Trying to put together a doorbell that plays audio using my Arduino. My current boring doorbell switches on 24VAC on the press of the button. I would like to use this to trigger a pin interrupt to wake my arduino so it can perform bell ringing duties. The question, how to I get that 24VAC to trigger an interrupt?

My ideas are:

1. Use a relay with an AC coil to pull a pin to 5V. My issue here is that I've only found 12VAC and 120VAC relays at Radioshack (didn't feel like waiting). I think I know where the 24V transformer is and I picked up a 12V one to replace it if that is an option, but I believe it's used for the house's HVAC as well, so I don't think that's an option. Am I missing something here? Should I just find a 24VAC Relay online?

2. Use a 50V 4A full wave bridge rectifier to turn the signal into 24 VDC and feed it to a +5V regulator and hit an opto-isolator to trigger the pin.

Ideas? Advice? I'm open to anything. Thanks!
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Re: Using 24VAC as an interrupt?

by adafruit_support_bill on Sun Nov 25, 2012 6:14 am

I don't know how your house is wired, but for doorbells, the power is typically wired to the bell/chime unit and then just a pair of wires out to the button to close the circuit. If you disconnect the button wires from the chime unit, you can wire the button to the Arduino as you would any other pushbutton.

If yours is wired with power to the button, you could have the 24vAC light a led and use a photo-sensor on the Arduino to detect it (a DIY opto-isolator).
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Re: Using 24VAC as an interrupt?

by jarruda on Mon Nov 26, 2012 8:38 am

It turns out that I mis-remembered how the circuit was arranged. That's what I get when I go months between having an idea for a project and actually following through on it :) The button is indeed supplied by the door chime box, and it has 16V (20V unloaded) being fed directly to it. This simplifies my project as now I won't have to worry about switching on 16V current or maintaining a low-power project running on batteries!

So now my plan is to just run the project by that 16VAC and switch on the doorbell button.

On running from 16VAC - I should be able to use my 50V bridge rectifier diode to an electrolytic cap, then a small ceramic cap to smooth output, then into the arduino's 5V reg should work to power the device (provided the transformer is high enough current), right? Maybe 2 diodes in series to drop the voltage closer to the linear reg's safe area?
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Re: Using 24VAC as an interrupt?

by adafruit_support_bill on Mon Nov 26, 2012 9:01 am

Rectified 16vAC is going to be too high for the 7805 on the Arduino (they run pretty hot at 12v as it is). You might consider using one of these to power the Ardino via the 5v pin. http://www.adafruit.com/products/1065
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Re: Using 24VAC as an interrupt?

by jarruda on Mon Nov 26, 2012 10:16 am

I didn't realize that you could bypass the regulator on an Arduino that didn't have a VCC pin! Drat. I really wanted to finish this project today, and I already wired up the circuit as I had mentioned, minus the final connection to Vin.

It sounds like if the 7805 fails it's a fairly safe device in that it shouldn't create a short or allow unregulated voltage to pass through. I think I'm going to try my luck and see what happens - hopefully the worst case being I'll pick up the buck converter and be able to power it via 5V pins.

Out of curiosity, does the amount of current consumed by the board have an effect on the energy that the 7805 must dissipate? e.g., If the board is asleep for 99% of the time, will the 7805 be running cooler because it's supplying maybe 30mA less power?
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Re: Using 24VAC as an interrupt?

by adafruit_support_bill on Mon Nov 26, 2012 10:28 am

does the amount of current consumed by the board have an effect on the energy that the 7805 must dissipate?

Yes it does. If you are sleeping most of the time you might be OK.
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