One of my long time interests has been to log my power usage. I have an ongoing project which will enable logging the whole house power consumption. That’s cool but I also wanted a way to log data for individual appliances. So I bought a Tweet-a-Watt starter kit (TAW) from Adafruit. This was a really cool solution to allow me to enhance a Kill-A-Watt power monitor (KAW) and make it transmit the power usage wirelessly so I could graph/log it.
But there was a problem… unfortunately the TAW didn’t work 🙁
After assembling everything, as soon as I would plug in the KAW, the screen would be blank and slowly come to life but still quite unstable and flickery. Data would start coming on the other end but after about 1 minute, the KAW screen would flicker really bad, then go blank and data would stop streaming. I tried a few things but nothing worked. The conclusion was that the XBee radio used by the TAW was drawing too much power and the KAW could not handle its needs. Apparently I had a newer revision KAW which was lower power which could not supply enough power for the TAW. Forums revealed that different revisions of the KAW had similar problems.
So I designed an alternative transmitter circuit using a low power Arduino clone that I designed for other wireless data logging, the Moteino. The operation principle is the same as the circuit from Adafruit. Power comes from the KAW through a Shottky diode to keep dropout low and stop current flow back into the KAW. There’s also a capacitor to help stabilize the circuit when it transmits. The Moteino runs at 3.3v so the current/voltage sampling from the KAW is fed to analog pins on the Moteino through a simple 10kohm<>4.7kohm voltage divider. An LED is used for transmission indication.
A python script was adapted from Adafruit’s version, to read data from a receiver Moteino, graph power and voltage/current waveforms, and also transmit power data to my Emoncms which is running on my RaspberryPi. The results are quite nice, you can see it in action in this video:
MikeT over at mikesmicromania.com posted his version of this project, this time including a Moteino!