Introduction: Add a Usage Monitor to a Home Thermostat

About: I've been taking things apart since I was 10. My mother wasn't impressed, even though I told her I knew how to put it back together... I've been making things since I picked up my first soldering iron (By The …

Long, long ago, way before there was such a thing as a "smart" thermostat, I had a home thermostat that gave a daily (I think - maybe weekly) total of "on time" for my heating and Air Conditioning system.

Things changed... The last time I shopped for a thermostat, I had a choice: A nice programmable thermostat at an affordable price but without a usage monitor, or an overpriced - and over-featured - "smart" thermostat, which I didn't want. I really missed that simple usage monitor, and spent months with the idea on the back burner of my mind.

What I wanted was something that would be inexpensive, compatible with a 24 VAC thermostat, be easy to power from the 24 VAC thermostat power, be self-contained with it's own display, and have non-volatile memory capable of recording at least several days of usage before rolling over or needing resetting.

At first I thought an Arduino-based data logger would be an ideal solution, and it probably still is, but after getting bogged down in the weeds of Arduino programming, 24 volt interfacing, the need for a continuous source of power, etc... Well, it went back on the back burner.
Recently, because I'd just had my AC repaired and was thinking about it, I re-visited the idea. Something made me look over at my little USB power meter I'd bought a couple of years ago for something like $5... Hey! This thing logs charging time, goes up to 99 hours, is USB powered, and has non-volatile memory!! Wow! Literally all I have to do is make it run on 24 VAC!

Well, almost all. We'll get to that.


  • A USB power tester. Don't get the kind with the LED display. It has to be one with an LCD display, like This one. It has to have a charging time display. Typically, these also display voltage, current, and total mAh, which in this usage, you can cheerfully ignore.
  • A 24 volt to USB buck converter. These are commonly used in cars to provide a USB port from 12 volts. Most will also work on 24 volts. Something like This.
  • An electrolytic capacitor rated at 35 volts or higher. The exact value isn't too important; I used a 1000 uF because that was what I had available. Anything 220 uF or higher will probably work. It's purpose is to filter the rectified DC after the diode.
  • A 1N4001 diode. Most any diode will work here. We're just using it as a crude rectifier, and it's going to be carrying very little current.
  • A 150 ohm resistor for use as a load.
  • Either an old USB cable you don't mind cutting up, or a USB plug that you can solder to.
  • A multimeter. Any cheapo will do. Harbor Freight gives them away sometimes.
  • Soldering equipment.

Step 1: Measure Twice...

I'd already done the preliminary work when I first conceived this idea. All that was needed was to find the two wires out of the four going to the thermostat that control the blower. That way, whenever either the heat or the AC was on, it would send voltage through those two wires to signal whatever I eventually came up with.

On my 4-wire thermostat - with a gas heater and standard AC system - the wire combinations are:

  • White - the common wire
  • Yellow: A/C
  • Green: Fan
  • Red: Power

I didn't test for the Heat wire, because I'm mostly interested in how much my A/C runs. This is Arizona, after all! (As in, "Snow? What's that??") If you live in, say, Minnesocold, then you might be more interested in heat, but the principle is the same.

Because of the way my thermostat is built. I couldn't just take the cover off it and start probing wires, because the cover is the thermostat, and the part attached to the wall is just a terminal block. I cut some thin wires and inserted them into the terminal block beside the wires already there, then led them out to where I could probe them after reassembling the 'stat.

When the blower is on, there is power between the white and yellow wires. That's what I need to know. Those two wires will be replaced with better wires, still leading outside the thermostat housing. I planned to just put my finished monitor on top of the thermostat, so I led the wires out the top of the thermostat.

Step 2: Theory and Practice

It's said that in theory, there's no difference between theory and practice. In practice, there is.

The first thing I did was plug my cool little USB tester into a USB port. Here was the only real snag in the whole project: The timer does not count time unless there is a load - in other words, something has to be drawing power from it.

Hoookay... We don't want to draw much power, because we don't know how much power the system has to spare. A small resistor that draws a few milliamps should do.

Again, I just happened to have a 150 ohm, 1/4 watt resistor in my parts box, and a USB cable with bare wire ends. I put the resistor between the red and black wires on the USB cable and Eureka! That, theoretically, should draw about 30 milliamps at the 5 volts that USB provides. In any event, it's enough to get the "clock" started, and the resistor won't get very hot. Be advised that a 100 ohm resistor will dissipate 1/4 watt of heat, putting it right at the top of it's rating. If you find you need a 100 ohm resistor, better get a 1/2 watt unit.

Because I had one, I installed the resistor in a USB plug for neatness's sake. The power terminals are the two outboard ones in a standard USB-A plug. If using a cable, it should be the red and black wires, but sometimes the Chinese cheapos use a strange color code. Check with your meter. Whichever two wires have 5V across them are the right ones.

On my unit, if the cursor between hours and minutes is flashing, it's counting.

Step 3: 24 VAC to 5 VDC

First, a little theory (Very Little!)

The standard for powering thermostats is 24 Volts AC. AC - Alternating Current, what comes out of your wall - is great for powering large and small motors, relays, heating elements, etc., but it's the kiss of death for electronics. Why? because it flows both ways sixty times a second, hence the name. To power a computer, radio, TV, etc., it must be changed into DC - Direct Current, what you get out of a battery.

It's pretty simple to turn AC into DC; a diode will do it. A diode functions as a one-way valve for electricity. Put a diode in an AC circuit and you chop off half of the AC wave, giving you pulsating DC. That's still not good enough for most purposes; we need to smooth it out. That's the job of the capacitor. The capacitor smooths out the DC, making it good enough for our purposes.

Resume normal behavior.

Refer to the diagram. Find out which input on the USB converter board is positive. Connect the capacitor across the inputs, making sure it is properly oriented. Capacitors have the negative lead marked. Positive to positive, negative to negative.

Now connect the banded end (very important) of the diode to the positive lead of the capacitor - or to the positive hole on the board if you can fit it in there. I couldn't, which is why it's hanging on the capacitor.

Now, those two wires from the thermostat? One (doesn't matter which) goes to the negative side of the capacitor, the other goes to the free end of the diode.

Step 4: Make It Pretty and Hook It Up

I 3D printed a little box for the USB converter assembly, to protect it and make it look better.

Now all that needs to be done is plug the USB power meter into the USB converter, plug the "load" into the meter, and you're done!

Now, every time the blower comes on, the clock will be running. If you know about how many amps your system draws, you can get a pretty good idea of your next electric bill. My system costs about 73 cents per hour to run. Add that to your off-season bill and you know about how much you'll be gouged.

One thing of note: It turns out that the timer on the USB stick does not "roll over" to zero when it gets to 100 hours; instead it reads "FULL," and will need to be manually reset. I also reset it monthly on my meter read days.

Modify It Speed Challenge

Participated in the
Modify It Speed Challenge