Not So Tiny Power Meter

Posted in Projects, Software Libraries, Tutorials by Bill
19 Dec 2010
Not So Tiny Power Meter

The Kill A Watt is an awesome product; it measures volts, amps and power factor of an individual appliance which can be used to calculate power, cost to run, etc. It’s also quite hackable. But I wanted something that would give me the same data for my whole apartment. After some Googling, the best I could find was this project from picobay, but I didn’t want to invest in an expensive network IO platform. There were also some off-the-shelf solutions, but they too were expensive and limited. Well, time to design my own solution then.

Enter what I call the ‘Not So Tiny Power Meter’. The catchy name comes from the microcontroller I used, ATtiny85, and some sizing issues I had with the enclosure.

I started out with a plan to use volt-meter current clamps just like the project I linked above (photo of clamp from and use a dedicated chip, the AD736, to convert the AC signal off the clamps to a DC voltage representing the RMS current value. The chips are expensive, tough to use as I found out, and still require external amplifiers to scale up the value to 5V ADC range. So I nixed that idea. Instead, I decided to use a single op-amp to scale up the AC voltage off the clamp and sample it directly with the ATtiny’s ADC. The circuit would be cheap and easy to design and I can convert the signal to RMS in code.

Then I had a thought. If I’m sampling directly, why not measure more than just amps? As an EE, I’d love to know more about my power usage, like power factor, frequency, and a more accurate measure of power by not assuming a voltage like most other projects; but I still wanted to keep the device simple. Then I had another thought: Why not measure voltage through the same transformer that’s giving my circuit power? After a few tests, I found that a properly designed rectifier and regulation circuit wouldn’t distort the source AC waveform too much. They key was to keep the values of the capacitors before the voltage regulator (circled in red) to a minimum, just enough to support a stable DC voltage. Anymore and the inrush when the rectifier diode starts conducting severely distorts the AC wave form.

My design is simple. An AC transformer powers the circuit and a voltage divider drops the source voltage down to ADC range for measuring. A dual sided half-wave rectifier and regulation circuit provides +5V and -5V rails. The AC signals off two AC clamps are scaled up using two op-amps. I planned on using trim potentiometers to calibrate the gains of all the measurement circuitry, but found it was easier to just use transfer functions (found with experimentation) in code.  Everything is measured with an ATtiny85, and transmitted out of the breaker panel by a cheap RF transmitter. Since all sources are AC, the ATtiny could only read the positive half of the waveforms. When the signal would go below the ATtiny’s GND, the protection diodes and input resistors would protect the ADC pins from damage.  With this design, I can measure voltage & frequency off one phase and current & power factor off both phases.

(Circuit Diagram, Click for Full Size)

The theory of operation is simple. First, the ATtiny85 will repeatedly sample the volts ADC pin for over a full period of the 60Hz sine wave. The peak value of the samples is remembered. Repeat for both current clamp ADC pins. After the max values are captured, the ADC clock is increased for faster sampling, with higher errors. To measure frequency and power factor, I used a 8 bit timer that  I extended to 16 bit in software. Using the timer, I measure the difference in time between two peak values of the voltage waveform. Then, I measure the difference in time between a peak value of the volts waveform, and a peak value of the current clamp waveform. Repeat for the second clamp. After all these measurements, some conversions are done to convert the peak values to RMS, times to frequency and power factor, run through a transfer function to account for various gains in the circuit and transmitted out via software serial as ASCII sentences with checksum.

Comparing to real measurement hardware, my project had respectable measurement accuracies of:

  • +/-  1 Volt,
  • +/-  1 Amp,
  • +/-  2% for frequency,
  • +/- .03 power factor when current is above 10 amps.

The circuit is designed for 120 Volt, 100 Amp mains, but can be adapted for other systems.

I ran into a few issues through the course of my project. The first issue was with the enclosure. All good projects should be protected by an enclosure, especially when installed into a breaker panel. First my poor planning resulting in a enclosure that was too small to house all the banana jacks for the current clamps. Then the second enclosure didn’t match the mechanical drawings provided by the manufacture. What stinks is I already had PCBs made to the spec of the drawings before I received the case. O well, time for double stick foam to mount the PCB instead inside an overly large box (part of the irony of the name).

The second issue was the quality of the signal off the current clamps. When using function generators for testing and programming, I could measure frequency, power factor and max value with great accuracy. The noisy signal off the current clamps is another story. Really, power factor measurements with currents less then 8~10 amps are very noisy.

Third, I originally used 434Mhz radios, until I realized it is the same frequency as my external temperature/humidity probe for my clocks. I quickly changed the radios to the lower frequency versions.

Anyway, I got the project built, tested, and installed into my breaker panel. Everything is internal so nothing extrudes. Right now, the data is received by another one of my projects, an Arduquee display. The display just shows live power usage. I plan to experiment with data loggers to log the data and/or play with the Google PowerMeter API to send the data into the cloud. This project was just to build a sensor to get the data out of my breaker panel.

Here’s some photos from the build, and the installation into my breaker panel. Click for larger pics.

Quite a workbench

Checking the signals

Installation into panel


The temporary LED display showing power (Watts), now hanging on my wall. Notice the RF receiver on the left.

My design is open-source. All the theory of operation is well documented in my code, and all code and Eagle PCB files are available to download:

Not So Tiny Power Meter files(zip)

I even have extra PCBs for any that what them, $5 plus shipping. Drop me a line in the comments if interested.

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  1. 135 Comments.

    • Rahul SarangleNo Gravatar says:

      I want to purchase this project.One question is this project will work for 230V , 50 Hz supply. Kindly send me the payment details.

      Rahul Sarangle

    • ElnurNo Gravatar says:

      Why you did not use simple ommeters to transfer data to Arduino directly? like theese ones: ACS712 30A or 200A Bi/Uni AC/DC Current Sensor Module?

    • AlvaNo Gravatar says:

      Exploring I noticed your web site book-marked as: Not So Tiny Power Meter

    • I would still like an extra PCB if you have one laying around. Would love to put this together at my house!

    • Phil ResmerNo Gravatar says:

      I run a screen printing business from home and use a ton of electricity .. I would absolutely love to buy this as a kit or even complete working product if you were willing .. I can’t afford to pay near $300 for the commercially ones unfortunately ..

      Please feel free to email me if you like Bill or I’ll check back here , thanks!

      • Salman SheikhNo Gravatar says:

        The boards can be bought for $44.15 for 3 pieces ($14.72 each) from See this quote see this site that I tried with the gerbers. Anybody interested in getting in with me to buy a set of 3? If we could chop off some of the bottom of the board (logo and texts) and make the square inches smaller, we could get it even cheaper..

    • Mark SNo Gravatar says:

      Is anyone still looking for boards? If so, reply here. I am willing to rework the PCB file to make it smaller and am looking to split up an order…

    • Mark SNo Gravatar says:

      Sorry, the summer just flew past. Bill’s comment made me come back to the site. I will try to find some time to work on the board shrink.

      • BillNo Gravatar says:

        Whoops, this is what I get for reading comments in the admin panel, I didn’t realize it wasn’t aimed at me!

      • I already attempted it..I was having problems with deleting the logo as it takes lots for real estate..but I figured out last week with another design when sending it to oshpark for fabrication (but my new logo didn’t appear on the fabricated design, oh well..functionality over aesthetics but both would be nice) I will search for that schematic and post it and we could collaborate..and Bill..very nice the geeky wedding…you are too’s my 19th anniversary but if I was getting married now, I would have to have some LED wear in my wedding..I’d probably have the first Islamic wedding with LEDs..LOL 🙂

    • DavidNo Gravatar says:

      Hi Bill
      Really thanks for your useful explanation.I am a final year Electrical Engineering Student @ uni.My area of my study is Power(high voltage),but i have to do a project in “power factor display”.The aim of this project is to design and develop a power factor controller using current and voltage transducers in addition to other circuitry. A visual display of the power factor value should be included.
      i am not good at electronic and this project seems to be lay on electronic.
      I don’t know how and where should I start.
      Could you please help me by showing me the basic of this and the process.
      many thanks

    • Aris MagklarasNo Gravatar says:

      Is it possible to install this smart meter in medium voltage(20kV)?If yes please send me an email about the installation.I am interested to buy this product.

    • DaveNo Gravatar says:

      Looks like a great and fairly simple project ,,do you have any pcb’s left?

    • daveNo Gravatar says:

      Noticed while compiling your c file that I got an error on the 2 lines that use sprint command .It compiles just fine if target board is arduino uno but errors out if target board is attiny85. Any advise.

    • […] Yani Tiny Değil Güç Ölçer – tüm daire değil Yani Tiny Güç Ölçer güç tüketimini T… […]

    • tolenna joeNo Gravatar says:

      what causes the dial on a meter to spin backward or counterclockwise?

    • alyaNo Gravatar says:


      Im a high school student living in UAE. And I’m working on a project of how to monitor electricity consumption at home. I have seen your work and think that its just what I’m looking for. Is there any way you could help by sending me instructions and details? is there any easier way of building it. please get back to me as soon as you read this. And I’m willing to pay if its going to cost anything.

      • SofianNo Gravatar says:

        Hi Alya
        Have you finished your project?
        I’am based in Dubai and also an EE

        This was a very nice inspiration for a new and more powerful project.

        Other side of the story
        He got married, forgot electronics…. where is he now Bill?

    • DeanNo Gravatar says:

      This is quite old but just as relevant, here is a bit of info on my take on the project.

      I have built up the board but not so sure on how to read the data stream.
      My goal is to read the output with an ESP8266 web server and display the power data from a web page that resides in the ESP8266. This data would then be combined with my roofs Photo-Voltaic production. The result would be displayed using small lamp that would display a corresponding color to the current over/under production and give me some indication as to when it might be time to turn on the pool heater or other high demand item and exhaust the use it or loose it over production.

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