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
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:
- Skip the first 100 frames (
-S 100) to give the webcam time to adjust its exposure levels
- 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.
Streaming the output
#!/bin/bash INTERVAL=5 WEBCAM_IMAGE="/home/lucas/webcam.jpg" CSV_FILE="/home/lucas/data.csv" SSOCR="/home/lucas/ssocr-2.16.3/ssocr" 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" done