Wireless MCUs and power consumption, part I

Electricity is expensive and being low power is environmentally friendly. And what is more motivating than seeing the total power consumption in real time? There are plenty of commercial products out there already but will they give you the raw data so that you can plot nice graphics? Not that many. And of course self made is always .. self made.

Lately in Finland the old electricity meters have been replaced with smart meters and they seem to have leds showing the power consumption. The model I have has a led that blinks once for every consumed watt-hour so it is easy and safe to calculate the power consumption by counting the blinks.

I am using a self designed CC430 based 433Mhz radio board and a simple photo resistor to count the blinks of the led in the apartment’s smart meter. Every minute it sends the count to another radio hooked into a Raspberry Pi that in turn sends the value to a server in the network. Then I have a simple javascript based web page that shows the data with a minute or two latency.

Power consumption over time
Javascript based power consumption info page.

There is no power plug that I could use for the radio board so it is running on two AA batteries. I put everything in a small plastic box. Below is an image of the box before I placed it next to the smart meter.

power_measure_unit-20140703

The radio part of the CC430 is turned completely off when not sending and the MCU part is sleeping. The only part running is the comparator that compares the photo resistor’s output to a predefined thresholds. If the threshold is exceeded an interrupt fires and I count the interrupts. Once a minute I reset the counter, turn on the radio and send the count over the air. Running the comparator takes some 200uA and I think I might be able to just bluntly interpret the photo resistor’s output as GPIO. That should drop the consumption below 10uA. Even with the higher consumption it has been running well for a few months now as can be seen in the graph below.

Battery voltage level
2xAA voltage level over time.

Having to replace batteries is inconvenient and another project of mine will be running outside so I can use a solar panel. More on that in Part II.

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