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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Trackbacks / Pingbacks

  1. Clamp sensor power monitoring - Hack a Day
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  4. DIY Power Consumption Monitoring System - Hacked Gadgets - DIY Tech Blog

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  1. 135 Comments.

    • IvanNo Gravatar says:

      Bill, that’s some good work there. I like it. Who makes your PCBs? What CAD software do you design them with?

    • BillNo Gravatar says:


      I use a site called SeeedStudio to fab the PCBs. They really just outsource in bulk to a cheap Chinese fab house. $20 for 10 PCBs under 5x5cm. $40 for under 10x10cm. Shipping is long unless you pony up for express UPS.

      And I use regular old Eagle to CAD them.

    • […] in the pursuit of an inexpensive yet effective way to monitor his power usage. He calls his project the Not So Tiny Power Meter, and shared both his successes and follies involved in seeing it through to implementation. There […]

    • RJNo Gravatar says:

      I’d be interested in buying one of your extra PCBs. Just the type of thing i was looking for.

    • PeterNo Gravatar says:

      I would be interested in one of your pcbs

    • […] fix. I could have just done software PWM, because the ATtiny85 isn’t doing anything else (like monitoring electrical mains) but in the end I just finished what I had started with […]

    • wcbzeroNo Gravatar says:

      I am also interested in a PCB.

    • RegNo Gravatar says:

      Is it just me? I find the clamp on meters very strange. A few turns of #24 around the feeds and a little extra effort on the MCU side would do this and be a lot cleaner and cheaper.

      I’d be pretty weirded out if I opened a panel and found multiple clamp on meters inside.

      Is reading an analog signal really that hard? A 16 bit A/D would give ~3 mA resolution on a 200 A feed. The MSP430 has 8×16 bit A/D channels, and I assume that many other MCUs offer similar peripherals on chip.

      BTW The form would not let me give my name no matter how many times I tried until I triggered an error. Then it allowed me to put it in.

    • […] in the pursuit of an inexpensive yet effective way to monitor his power usage. He calls his project the Not So Tiny Power Meter, and shared both his successes and follies involved in seeing it through to implementation. There […]

    • rezaNo Gravatar says:

      I’ve been working on a similar setup using some current probes that output a nonlinear voltage for a given current. I didn’t like the design, and am in the process of replacing the circuit board in the current claps with a simple resistor+diode in parallel. They’re current coupling clamps so I get gain by just using the right sized resistor.

      The plan is to sample the waveform and compute the RMS value. The question I have for your approach is that it seems to assume a sinusoidal waveform. The way electrical dimming units work (Triac-based circuits) you end up with funny shaped waveforms and I don’t think it’s sufficient to just measure the peak value. I also don’t want to tap into the power lines directly so I’m not measuring voltage or anything. How did you do it? And how did you calibrate your results?

      otherwise, looks like a nice project.


    • cliff52No Gravatar says:

      On a single phase service, the power legs are 180 degrees out of phase. That means if you can arrange the power leads in opposite directions through a clamp-on ammeter or current transformer, you can measure the sum of the currents with one device.
      Great project – thank you for sharing the details.

    • BillNo Gravatar says:


      Yes, I am assuming a sin wave form. So for me, it’s just peak value * .707 for RMS. If you look through my code, you can see how i just captured the peak value. I’m not familiar with what a Triac will do to the signal however. Do you have an example of what it would look like?

      I calibrated the device one leg at a time. With two clamps on the same phase, I compare what my device was reading off one clamp to what my trusty voltmeter was reading off the other.


      One way to do it, but you still would have to use high precision resistors to measure the current accurately. And I think any 3rd party would find clamps just as odd as thin wiring running about too. Clamps are $10 a pop and are already calibrated voltage output sensors, so they were easy to use.

      Did I ever say reading ADCs were hard? and the chip I’m using has 10 bit ADCs FWIW.

    • BillNo Gravatar says:


      Hmm, didn’t think of that, good idea. Though in my case there’s no way I’m getting the two main runs near each other to run them through the same clamp like that. I almost didn’t have enough slack on them (or strength on me) to move them were I needed just to get a clamp on each one.

    • […] Porter has designed and built a DIY Power Consumption Monitoring System which uses an ATtiny 85 microcontroller to keep track of power usage in his house. He is using two […]

    • elettronicoNo Gravatar says:

      Cool! Awesome project and awesone title, kill-a-watt is cool too

      Thank you

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