24vac to 5vdc sense

[Jeff Johnson]

Guys/gals, if I want to read a 24 VAC signal on whether it is on or not and read it with an Arduino.  When I google this, all I get is power supply stuff.  Can you circuit jockeys look at this and let me know if this would suffice, or if not, can you provide a better option?

Thanks.

Jeff

[Joe Freeman]

Your circuit there is essentially a half wave rectifier. I would put the rheostat on the output side of the diode for the below reason-

As I recall, your output voltage in this case will be (2 * (24v * sqrt (2) -  .7))/pi, or about 21.6vdc, ignoring the ac ripple and assuming 24 VAC rms input.

You will need a voltage divider network on the output side of the diode. I would put the rheostat on the output of the rectifier so you could adjust the output voltage by using it as an adjustable voltage divider.

I would probably try to find a higher value rheostat (more ohm) to limit the current in the circuit as well.

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[Jeff Johnson]

The 10K thing is a potentiometer.....  or is that the same thing as a rheostat?  I was thinking I could adjust the pot until I got a around a 4.5 to 5 v output.

The pot would only need to dissipate 0.0576 watts (if my math is right (i^2*r) )

Thanks!

[Jason Brown]

It sounds like you are wanting to sense a thermostat or similar circuit. If so, have you found a common ground somewhere to sense against?

Sensing with 1 wire, and no common ground / reference would require a current transformer or something similar.

[Jeff Johnson]

Very astute of you, Jason (as always).  Yes, I was thinking of watching my tstat to log inside/outside temps and how often it comes on.  In particular, I would like to eventually make a device that sits between my tstat and HVAC, so when the house is vacant (vacation mode) it activates the condensor and house fan during opportunistic times.  For example, during the winter if the heat pump is set to 40 in the house, and a warm day comes along, say 50 to 60, I want my system to run and heat up the house when the efficiency is high.  And likewise, run my system during the summer when the temp drops to 60 at night.  Of course, using outside air would be best (like an economizer does) but I don't want to do that much ducting.

But before I do something like that, I want to log  how much my system runs now along with the outside and inside temps, and to do that I need to know what the tstat is doing.  On the tstat is a 24ac common (black) that I was going to use.this as my ground.

[Jeff Johnson]

What I would eventually LOVE to do, is to make a R-pi based programmable thermostat.  And I don't mean programmable tstat in the traditional sense, but rather one you can actually program in C to do cool things such as checking the internet for weather changes, or emailing me when filters need changing, or changing temperature when I send it a text or email.

If anyone ever wanted to help with that, I'd be willing to fund it.  It would be a product to sell on Ada or Sparkfun.

[joshua johnson]

If you're just trying to sense on/off, why not use a relay?

[Jeff Johnson]

well..... I guess that would work.  Hadn't thought of that!  It would add some isolation too from any spikes.

[Jason Brown]

Relay: Will definitely work. A relay is a nice simple option for quick and dirty, and safe for the sensing circuit.

Jeff: That common lead: the 24vac transformer that feeds your thermostat has two output wires.  One is connected to the "C" terminal on your thermostat, the other is the one switched to each control system. AC power is on both of those lines. Tying an arduino ground to that line would most likely fry the arduino since it is powered. Using a relay makes this a moot point and the HVAC system and arduino do not need to share any signal paths.

For data logging, you could also use a current sensor in your breaker panel and record actual energy usage for the circuit (openenergymonitor or similar). 

Just the first sensor I found: https://www.sparkfun.com/products/11005

For thermostat work, take a look at some of these projects for ideas and inspiration!

http://blog.spark.io/2014/01/17/open-source-thermostat/

http://openenergymonitor.org/emon/

http://androidthermostat.com/

http://www.instructables.com/id/Introducing-Climaduino-The-Arduino-Based-Thermosta/

http://wyattwinters.com/rubustat-the-raspberry-pi-thermostat.html

http://www.element14.com/community/community/raspberry-pi/raspberrypi_projects/blog/2014/01/20/raspberry-pi-and-pifacecad-board-thermostat-project

http://ve2zaz.net/RasTherm/RasTherm.htm

--Jason

[Jeff Johnson]

Thanks Jason, et al.  I actually have that current sensor from Sparkfun (bought it from Sparkfun).   I'll be just as happy to know from a percentage basis.  I have a adafruit datalogging shield that I'll probably use, as well as some temp/humidity sensors.  If I ever get around to making it, I'll try to document it for the site.