Hacking together a cheap realtime power usage monitor

I’m currently in the throws of hacking together a bunch of old laptops into a beowulf cluster, so when I wanted to monitor the cluster’s power usage on my computer instead of paying out for a new-fangled gadget I did what I’ve been doing for the cluster and delved into my box of old electronics.

After a bit of rummaging (and online searching) I managed to bodge together the following:

  • Power Meter Adaptor (LCD)
  • Webcam (EyeToy for PlayStation 2)
  • fswebcam - Captures webcam images
  • ssocr software - Seven-segment digit recognition

Webcam attached to power usage monitor

The assembly of the contraption was fairly straight forward. I securely attached the webcam to the power meter via a cardboard frame, extended the webcam’s LED to light the power meter’s LCD and connected the whole thing to a computer with Ubuntu (which comes with the UVC driver).

Capturing the LCD readout

Capturing an image of the display was simple enough with fswebcam, which I’d configured to:

  1. Skip the first 100 frames (-S 100) to give the webcam time to adjust its exposure levels
  2. Combine 5 photos taken in quick succession (-F 5) to reduce the noise
$ fswebcam -r 640x480 -F 5 -S 100 --no-banner --save ~/webcam.jpg

Recognising the digits

The obvious problem with using a segmented digital display is the recognition of the seven segment digits, however there is a great program called SSOCR that does this for you - and even better it also has options to help prepare the image.

$ ssocr -t 10 -d -1 rotate 3 crop 255 234 200 140 erosion ~/webcam.jpg

The options used in the command are listed on SSOCR’s homepage.

SSOCR image manipulation of LCD screen

Streaming the output

By automating the aforementioned tasks the following bash script stores the readings in a CSV file, which my cluster’s Dashing dashboard can then tail and display as a colourful graph.



while true; do
    sleep $INTERVAL

    fswebcam -r 640x480 -F 5 -S 100 --no-banner --save $WEBCAM_IMAGE > /dev/null 2>&1

    READING=$("$SSOCR" -t 10 -d -1 rotate 3 crop 255 234 200 140 erosion $WEBCAM_IMAGE)

    echo "$(date +%s), $READING" >> $CSV_FILE
    echo "$(date +%s), $READING"