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I have a appliance that is switched with a 24ac signal.  I want to use the arduino to count the number of times it turns on and off during a giving day.   I'm just not sure how to tie the 24ac signal to something arduino friendly?  Any help would be great
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AC VOLTAGE TRANSDUCER

It will tie the 24ac signal to something arduino friendly, however it is not very price friendly.
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Does it have an indicator light you could slap a light detector on?

Failing that how about a bridge rectifier into an opto-isolator via a couple of current-limiting resistors?

There's a circuit on this page that shows detecting the zero point of a 230V AC signal. Now 24V is somewhat less than that so the resistors would be different but the concept is the same.
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I just tried this circuit out on a 15V AC transformer I had lying around. This is going to be similar to your 24V AC device.



You could use a bridge rectifier (as I did: W02M) or 4 x diodes to rectify for the opto-isolator. The 2K resistance (in total) would reduce the current to around 12 mA which is about right for the device. That completely isolates your Arduino from the appliance. Then on the other side the opto-isolator turns on when the current flows, driving the output low. The capacitor is there to smooth out the AC signal so it just looks like an on/off situation, not on/off 50 or 60 times a second.

Note that the output (to Arduino) is low when the device is on, and high when the device is off.

You could probably eliminate three of the diodes because half-way rectification would be adequate in this case.
« Last Edit: March 22, 2013, 12:00:47 am by Nick Gammon » Logged

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I might be wrong about the half-rectification. After all we are trying to discharge the capacitor, and if we only activate the opto-isolator 50% of the time, the average would be for it to stay (at least partly) charged.
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Quote
You could probably eliminate three of the diodes because half-way rectification would be adequate in this case.
Yes we could, all we need add one electronic capacitor at output half-way rectification.

or use this



since we face 24V instead of 110V, cap could be droped.

second one use Optocoupler with AC Input.


« Last Edit: March 22, 2013, 11:56:24 am by sonnyyu » Logged

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Would something as simple as a single diode (makes 1/2 wave DC), a 10uf cap(turns it into lumpy DC) and a 78L05 regulator provides protection and makes very nice DC be appropriate ? (looking to minimal component and cost) Does this approach provide isolation?
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I might be wrong about the half-rectification too.

Since u have one electronic capacitor connect with Arduino input. electronic capacitor at output half-way rectification might not necessary.
« Last Edit: March 22, 2013, 10:09:24 am by sonnyyu » Logged

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Would something as simple as a single diode (makes 1/2 wave DC), a 10uf cap(turns it into lumpy DC) and a 78L05 regulator provides protection and makes very nice DC be appropriate ? (looking to minimal component and cost) Does this approach provide isolation?

No
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Thanks for all the responses!  I was thinking of rectifying it, just wasn't sure what to do beyond that.
I will definitely play with that circuit
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