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.

    • BillNo Gravatar says:


      I cut the socket end off an extension cord, stripped and wired it to get power off a breaker. Not exactly to code, but it works.

    • HectorNo Gravatar says:

      You talked about a kit and also about upgrading to an ATmega.
      Still a possibility this kit or the idea was abandoned?
      Thank You for the answer

      • BillNo Gravatar says:

        Still a plan, i’m just boged down by other duties at the moment so progress is slow. Mostly by the childrens outreach I do. See ‘Who is bill porter’ link above.

    • How about putting version 2 on an arduino shield?

    • Chad GibbsNo Gravatar says:


      I may be a little late but I would love to have a PCB board if you have any left. I am really excited to play around with it.

    • steve greensillNo Gravatar says:

      Hi Bill,

      I would love 2x pcbs if you have any left.

      – porting this to an arduino shield would be great!


    • […] no construirlo porque lo sensores de corriente son muy caros. Te dejo links a proyectos que pille: Not So Tiny Power Meter The Mind of Bill Porter An AVR-based logging wattmeter PowerBox: The Safe AC Power Meter Home energy monitor | […]

    • Justin ShattuckNo Gravatar says:

      Hey man, I am still pretty new to hobbyist electronics, and well electronics in general. However I do have access to a Shopbot 4’x8′ CNC mill, Epilog Laser 40w and an Eden 330 / Eden 550. I could easily make a custom enclosure for you, and then make silicon molds for future builds. let me know,

      But I do need (a) any PCBs you might have available and / or a good place to get some pre-made like you did.


    • PaulNo Gravatar says:

      Hello, I would like to get two PCB’s – I can do paypal – Thanks, Paul Turner

    • Great Job! Can I also buy two boards? I would love to experiment with this project !

    • NachoNo Gravatar says:

      Hi Bill!

      I’m really impressed about your project. I would like to reproduce it. But I live in Spain, so… in order to adapt it to monophasic, 220V and 50Hz mains, which changes are needed? Could you tell me something about it?


    • KuhendranNo Gravatar says:

      Hi bill, i’m really interested in your project….can you kindly please mail the component list of this project

    • Rod da SilvaNo Gravatar says:

      Hey Bill,

      Just stumbled over this blog entry. Really enjoyed it. Do you have any PCB boards left? If so I will take a couple.

    • SherwoodNo Gravatar says:

      Knowing the real time power usage is a good first order solution.

      But we need a way to break it down.

      My vision is a small circuit clamp. It gets it’s power from the induced current from it’s own measurement. It has a small radio tranceiver. It has a very small amount of flash memory.

      It only operates when there is enough current to power it. The current taps charge a capacitor until it reaches a high enough voltage for it to ‘fire’ and it then can send some data.

      One of the things coded into the critter is a number — milliseconds after hearing a ‘sound off’ signal to wait before broadcasting it’s data.

      A more central device periodically sends out queries, and listens for the responses.

      The unit should be no larger than a two nickles taped together, with a 12 inch antenna tape. Small enough that you wire them into the outlet box, or the light switch.

      Two kinds of devices — one that measures the current, one that merely detects that device is on.

      They have to be cheap. $5 each in quantity.

    • Richard ArellanoNo Gravatar says:

      Would appreciate two power meter boards. Am interested in monitoring a 48K BTU heat pump. Here in the Texas Hill Country, last year we experienced well over 100 days of 100 deg. F. heat.
      The unit is two stage and am curious…

      BTW, my first faculty position was at FSU in the B School – 1964, as I recall. Dean Charles Rovetta, a great mentor, asked me to design and implement an International Business program. I also steered the Honors Program, and still remember some of the most inquisitive and sharp students that I’ve ever encountered.

      PayPal O.K.?

    • Will ParksNo Gravatar says:

      Hello, great project. I was wondering if you still had 2 of the circuit boards left?

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