Taikapeili1
Here's how I made glanceable wall mounted weather display from an old Android tablet. The display is motion activated and shows a weather forecast, the UV radiation index for the day and an air quality estimate.
- Nexus 7 (2013) Android tablet.
- Xenon ZigBee smart plug for controlling charging.
- A web app that displays the weather (i.e., this repository).
- Chart.js for rendering charts.
- Fully Kiosk for running the web app on the tablet.
- Home Assistant automation for turning the charger on and off based on the charge level.
To save power and reduce the stress on the battery, I wanted to automatically turn off the tablet overnight when it isn't needed and maintain a 65-75% state-of-charge on the battery.
I couldn't find any kind of scheduled shutdown functionality on the version of Android on the tablet, so I opted to flash LineageOS with root access so I could write a script to shut down automatically.
To power off the tablet at 00:30 every night, I created
/data/local/userinit.sh with the following contents:
#!/system/bin/sh
(while true; do test $(date +%H%M) -eq 0030 && /system/bin/reboot -p; sleep 30; done) &
exit 0This script is automatically run by Android at boot.
(Don't forget to chmod +x /data/local/userinit.sh).
If the charger is connected when the device powers off like this, Android will helpfully start up again to display charging status, defeating the whole purpose of turning off the tablet. Luckily this can be disabled with:
fastboot oem off-mode-charge 0If I had a more modern device, I could've used something like ACCA to set charge limits right on tablet itself. However, the Nexus 7 doesn't support software charge control, so this rube goldbergian contraption is the next best thing :)
The tablet is plugged into a Zigbee-enabled smart plug, which is connected to Home Assistant. I also installed the Home Assistant Android app on the tablet so that it can monitor the battery level.
With this, I only needed a pair of automation rules to turn the charger on when the battery drops below 65% and turn it off again when the level is above 75%:
I also added another rule to turn off the charger overnight and two others to start and stop the charger for 10 minutes in the morning to trigger the tablet to boot up again.
The graph below shows how this maintains the battery between 65-75% during the day and keeps the tablet powered off during the night:
(Why does the tablet use so much power when on? tl;dr: It's the camera-based motion detection to only turn the screen on when someone is looking at it.)
The web app is a single HTML page that displays the following information:
- Weather forecast for the day (from https://weatherwidget.io/)
- UV index prediction for the day (from https://uvdata.arpansa.gov.au)
- Air quality measurement (from https://www.epa.vic.gov.au/, requires CORS bypass and an API key)
- Rain radar (from http://bom.gov.au, requires CORS bypass)
These data sources are mostly specific to Australia, so you'd need to find equivalent services for your location.
Note that since some of the data is scraped directly from the source websites, Cross-Origin Resource Sharing (CORS) must be disabled in the browser hosting the web app. I achieved this by using the Fully Kiosk app.
First, I copied the web app files to the tablet with adb:
adb shell mkdir -p /sdcard/taikapeili
adb push index.html old-js/*.js /sdcard/taikapeili/
Since the browser in this version of Android is so old, I had to use
Babel to rewrite some of the more modern JavaScript
features used by the app -- see transpile.sh.
After this I pointed Fully Kiosk at file:///sdcard/taikapeili/index.html with
the following settings:
- Reload the web app every 10 minutes.
- Use camera-based motion detection to light up the screen only when someone is looking at it. I dropped the motion detection framerate to 1 Hz to somewhat reduce the CPU load.
- Starting Fully Kiosk automatically at boot.
Footnotes
-
Taikapeili is Finnish for magic mirror. ↩