+
++With this project, I monitor the temperature and humidity in my room. +Measurements are taken every 10 minutes and saved in an SQLite database. +Additionally, the display shows the upcoming appointments of the next five days: each of the 24 rows represents one hour and each pixel in a row represents 5 minutes. +In the middle, the time until the next appointment, the relative humidity and the current temperature are shown. +Below that, a graph shows the temperature of the last two days. +When activating another display mode (not active in the picture), this area of the display instead shows the next seven appointments. +
+Hardware
+-
+
- computer: Raspberry Pi 0 W +
- temperature and humidity sensor: AM2302 (wired DHT22) +
- OLED display: 1.5 inch SSD1351 (Waveshare) +
- yellow push switch: TRU Components PBS-18B 701912 +
+Wiring of the SSD1351 to the Pi (pinout): +
-
+
- VCC ↦ 3.3V +
- GND ↦ GND +
- DIN ↦ SPI0 MOSI (GPIO 10) +
- CLK ↦ SPI0 SCLK (GPIO 11) +
- CS ↦ SPI0 CE0 (GPIO 8) +
- DC ↦ GPIO 25 +
- RST ↦ GPIO 27 +
-
+
- VCC ↦ 5V +
- second pin ↦ GPIO 26 +
- third pin ↦ (unused) +
- GND ↦ GND +
Software
+ ++The Raspberry Pi 0 W is running Raspberry Pi OS 11 (bullseye). +It is advisable to enable SSH and configure a WLAN network before booting the RPi for the first time. +As explained in many tutorials online, this boils down to modifying the SD card image. +Create an empty file /boot/ssh to enable the SSH daemon, and configure the credentials of your WLAN network in /etc/wpa_supplicant/wpa_supplicant.conf. +This blog post describes how to set up the initial user and password. +Make sure to set dtparam=spi=on in your /boot/config.txt, otherwise the SPI connection to the OLED display will not work. +
+ ++All of the following software is written in Rust and available on GitHub. +take_measurements requests the current temperature and humidity from the sensor. +To mitigate transmission errors, the median of five readings is used. +The final reading is saved in the specified SQLite database (mine is named sensors.db) along with the current time. +refresh_json downloads the latest calendar entries from my Trilium Notes installation and saves them in a JSON file (events.json). +status_check gathers some miscellaneus status information on my other computer systems. The provided example just checks whether GitHub and GitLab are up. +display_all processes all of this data and displays it on the OLED. +It is run by another Python script (turn_on.py) whenever the push switch is pressed. +
+ +Below is the crontab of the pi user. status_check is run every five minutes. take_measurement is run every ten minutes. Fifteen minutes into the hour, any new calendar entries are synced using refresh_json. To avoid OLED burn-in, the display is turned off every minute (if it was on previously). ++*/5 * * * * /home/pi/status_check /home/pi/sensors.db 2>/dev/null >/dev/null +*/10 * * * * /home/pi/take_measurement /home/pi/sensors.db +15 * * * * /home/pi/refresh_json --no-weekly +* * * * * /home/pi/display_off off ++ +
+For performance reasons (and because the Pi doesn't have enough RAM), I cross-compile these programs using Nix. +I'm certain there's a better way to do it, but the following method works for my purposes: +in my NUR packages, run the following command to cross-compile all binaries: NIXPKGS_ALLOW_BROKEN=1 nix-build -A raspi-oled-cross. +(Btw, this will first compile LLVM and rustc for some reason. There must be a way to avoid that...) +As always, ./result is a symlink to the build artifacts. +The binaries (and the libraries they depend on) may be copied to the Pi like this: +
+mkdir /tmp/nixstore +nix copy --extra-experimental-features nix-command --extra-experimental-features flakes --no-check-sigs --to /tmp/nixstore $(readlink -f result) +rsync -r --links --info=progress /tmp/nixstore/nix pi@himbeere-null:~/ ++On the RPi itself, the binary needs to be adapted to Raspbian and placed in the proper position: +
+patchelf --set-interpreter /lib/ld-linux-armhf.so.3 nix/store/*-raspi-oled-*/bin/display_all +sudo mv nix / ++ + + +