Sensing AC signal from 24VAC thermostat transformer

[Roger2016]

I want to log my furnace activity, to sense & log when the thermostats are active, when the circulater is running and when the burner is firing.  I run a wood stove and in short my goal is to try to determine how much oil I spend on heat vs hot water.  So basically I want to sense signals that are 24 Volts AC.  No doubt the Motino would not be happy to see that voltage on a pin.  What's the best way to turn that 24VAC signal into something the Motino can handle?

Thanks!

[Felix]

Here's how I sense 16VAC on the DoorbellMote:
When 16 VAC is flowing from the doorbell the H11AA1 emits a half wave AC signal through a photo transistor which you can pick up and invert and convert to DC using a PNP transistor + smoothing capacitor and read on a digital pin.
You can do the same for 24VAC.
See this blog post which describes the DoorBellmote and also the initial theory of operation in this blog post.

[TomWS]

Odd rendition of a PNP transistor.  I suspect that the emitter and collector leads are reversed (although I suppose it could be operating in inverted mode but that configuration is rare and probably not necessary for this application...)

Tom

[damonb]

Symbol is NPN, so I think more likely it's NPN as emitter follower and the PNP label is wrong.

Come to think of it, the optocoupler would generate a stream of pulses at 60 Hz... Hence the emitter follower to charge the 10uF cap.

[Felix]

Thanks for keeping me honest guys, it's a PNP for sure. The symbol was wrong. I took the shortcut to use an existing Eagle part instead of creating a new symbol.
Schematic above and blog post are updated.

[Roger2016]

I can't seem to find that H11AA11 anywhere for sale.  Is there an alternate part#?

[sparky]

It appears DigiKey has the SMD option;
http://www.digikey.com/product-detail/es/vishay-semiconductor-opto-division/H11AA1-X007T/H11AA1-X007TCT-ND/4895860

Edit:  They do have an thru hole option also;
http://www.digikey.com/product-search/es?keywords=H11AA1-X006

[Roger2016]

I  was searching on    H11AA11 as stated in the post.  You found H11AA1 which looks correct.  Was the  H11AA11 a typo in Felix's post?

Thanks!

[Felix]

Whoops sorry again another typo, yes H11AA1.
Lately I've been using the LTV-814H instead, same thing, a little cheaper.

[sparky]

Whoops sorry again another typo

I think you just making sure were awake.. 

[Felix]

@sparky,
Ha, thanks for being nice and patient with me
I have my hands and brain in several different places most of the time, even when I (try to) post here.

[Roger2016]

Thanks so much for the help!

How about if all I wanted to do is to light up a LED to indicate locally that a zone was activated, could I just add the proper value resister in series with a LED.  since it's AC the led would be blinking at 60 Hz, but should appear to be on solid, or is there more to it?

Thanks again 

[Felix]

How about if all I wanted to do is to light up a LED to indicate locally that a zone was activated, could I just add the proper value resister in series with a LED.  since it's AC the led would be blinking at 60 Hz, but should appear to be on solid, or is there more to it?

The "RED LED" in the schematic above does just that.

[perky]

This is a neat solution, although I would be tempted to put a reverse diode across the opto-isolator LED and drive it from both AC signals, that way you don't need a common ground reference between your AC and local ground.
Mark.

[Felix]

@perky,
Not sure I'm fully following, the "GND" and "16VACSWITCHED" are the actual AC signals, the "GND" one functions as a ground for the circuit, the other AC signal is wired through a relay.