Pightstand

A raspberry-pi based nightstand clock

Summary

A nightstand clock as a programming project.

Link to original

Parts

Hardware

Item	Note	Link
raspberry pi	model B+ (40-pin); latest model (in link) is much more capable, same price	Adafruit
PaPiRus Screen HAT	2.7" screen, 4 buttons	US UK
microSD card	16GB SanDisk (boot media)	Amazon
USB WiFi adapter	Raspberry Pi official version; newer boards have embedded WiFi	Amazon
USB power supply	5V 2A recommended	Adafruit

Software

Library	Link
PiSupply Papirus	papirus
Python Imaging Library	PIL
Python API for WeMo	pywemo
Python client library for 3M50 Radio Thermostat	radiotherm
Python Requests library	native package (online docs)

Description

General Principles

• write code in python
  • this is, among other things, a programming exercise
• cache data from unreliable sources
  • connections to remote APIs are not guaranteed
  • when imaging the display, pull from cache
  • use separate routines to update cache
  • in this application, having old data is better than having no data
• use in-memory filesystem (/dev/shm) for temporary files
  • prevent wear to fragile SD cards
  • reboots wipe this filesystem, and that's okay
• use JSON to format data at rest
  • single standard format simplifies troubleshooting
  • yes, you can create JSON using shell scripts
• create separate programs for each task (a la microservices)
• leverage OS-level tools (bash, cron, cal) where appropriate

papirus-temp

This is a shell script I wrote to read the on-board temperature sensor. It provides the board temperature, which is regularly higher than the room temperature, and not much use in this project. Merged with the vendor repo (see history, below). Since then, a python version has also been contributed.

papiswitch

This is a python program that monitors the buttons on the PaPiRus Screen HAT:

• checks a PID file to make sure it's not already running
• event loop waits for keypress
• keypress is de-bounced
• first detected WeMo plug is toggled
• quits on error

A cron job launches this every minute. While not perfect, works 90% of the time (when I push a button, the light toggles). It's been suggested that I squirt XML directly to the device's IP address and skip the library, but I'd rather understand what's going on. This will need to support more WeMo devices in the near future.

papirus-image

This is a python program that loads the image on the e-ink display once it is generated by papigen, below.

• Full refresh on boot (timestamp file missing)
• Full refresh at top of hour (flashes screen, takes longer)
• Partial refresh all other times (smooth update)

papigen

This is the core, pulling together various data and formatting the display. This is also executed via cron every minute.

• polls my thermostat for current house temperature
• polls openweathermap every twenty minutes for outside temperature near me
  • Note: API key and location data have been sanitized
  • You can derive the coordinates of your device manually from Google Maps
  • Or automatically from your public IP address
  • Or dynamically by adding a GPS dongle
• executes the GNU cal utility to generate a calendar with a predictable text layout
• uses Python Imaging Library for screen layout (fonts, sizes, positions)
• creates bitmap image file for loading to display (by papirus-image, above)

crontab

In English, the stanza below:

• checks to see papigen is executable
  • if it is (logical AND), execute papigen
  • if it excuted cleanly (exit code 0), also execute papirus-image
  • dump all output (including errors) in the garbage
• checks to see that papiswitch is executable
  • if it is (logical AND), execute papiswitch
  • dump all output (including errors) in the garbage

The cron package runs this once a minute, per the first five asterisks in each line. If you put these lines in a text file (let's call it picron.txt), you can load it directly with the command: crontab picron.txt, and inspect it with crontab -l.

# m h  dom mon dow   command
* * * * * [ -x bin/papigen ] && bin/papigen && bin/papirus-image >/dev/null 2>&1 
* * * * * [ -x bin/papiswitch ] && bin/papiswitch > /dev/null 2>&1 &

History

Date	Note
9 Feb 2016	ordered papirus from pisupply
20 Mar 2016	submitted merge request for papirus-temp to PiSupply
17 Nov 2018	started nightstand clock idea instead of shoveling snow; initial commit
15 Dec 2018	pressing button toggles wemo lightswitch
4 May 2019	sanitized, commited to public repository