not so tiny power meter

Posted on Feb 6, 2014

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

not so tiny power meter
Click here to download the full size of the above Circuit.

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 picobay. com) 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...

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