I’ve been pretty curious how much energy the mirror uses. However, it’s taken me a long time to figure out how to measure it. For unrelated reasons, I’ve got an arduino that I figured I can use for the voltage measurement.


The power to the Arduino comes straight from the batteries, so I put a little voltage sensor across the battery feed’s positive + negative. It sends an analog signal to the arduino, and the arduino parses the analog input and sends sends logging info to the Raspberry Pi via a serial connection (note that the Arduino is set to 3.3v! otherwise the serial connection could damage the Pi).

There’s a daemon running on the Pi that parses the Arduino’s serial feed and writes tiny data files to a tmpfs dir. Each time the data reading daemon runs, it writes a file to the tmpfs with one line of data (actually, it writes a temp file then renames it; renaming is atomic, and so avoids a race condition between the serial parser and the data readers). Another daemon reads the data line and logs it. The two-part process means that one daemon can parse the serial data while several others consume it.

In this case, we want the log of the voltages. I set up the test with a single fully-charged battery (a 12v/100Ah deep cycle). Here’s what I found:


The mirror ran for over day! That means that having two batteries that are charged by solar during the daytime is much more capacity than we need to run it for 10 hours at night. Good.