<h1id="header"><imgsrc="favicon.ico"id="favicon"/><ahref="/">FliegendeWurst's corner of the WWW</a></h1>
<h2>Raspberry Pi 0 W + sensors + OLED display = temperature monitoring and calendar</h2>
<article>
<imgid="hero-img"alt="OLED display showing temperature and upcoming appointments, connected to a Raspberry Pi"src="./raspberry-pi-temperature-monitoring.jpg"/>
<p>
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.
</p>
<h3>Hardware</h3>
<ul>
<li>computer: <ahref="https://www.raspberrypi.com/products/raspberry-pi-zero-w/">Raspberry Pi 0 W</a></li>
<li>temperature and humidity sensor: AM2302 (wired DHT22)</li>
<li>OLED display: 1.5 inch SSD1351 (<ahref="https://www.waveshare.com/wiki/1.5inch_RGB_OLED_Module">Waveshare</a>)</li>
<li>yellow push switch: TRU Components PBS-18B 701912</li>
</ul>
The components are connected to the Pi as indicated in the wiring diagram below.
<embed
type="image/svg+xml"
src="circuit.svg"
title="Wiring diagram" />
<p>
Wiring of the SSD1351 to the Pi (<ahref="https://pinout.xyz/pinout/spi">pinout</a>):
<ol>
<li>VCC ↦ 3.3V</li>
<li>GND ↦ GND</li>
<li>DIN ↦ SPI0 MOSI (GPIO 10)</li>
<li>CLK ↦ SPI0 SCLK (GPIO 11)</li>
<li>CS ↦ SPI0 CE0 (GPIO 8)</li>
<li>DC ↦ GPIO 25</li>
<li>RST ↦ GPIO 27</li>
</ol>
Wiring of the AM2302 to the RPi:
<ol>
<li>VCC ↦ 5V</li>
<li>second pin ↦ GPIO 26</li>
<li>third pin ↦ (unused)</li>
<li>GND ↦ GND</li>
</ol>
Wiring of the push switch to the Pi: one pin to GPIO 19, the other to 3.3V.
</p>
<h3>Software</h3>
<p>
The Raspberry Pi 0 W is running <ahref="https://www.raspberrypi.com/software/">Raspberry Pi OS</a> 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 <spanclass="code">/boot/ssh</span> to enable the SSH daemon, and configure the credentials of your WLAN network in <spanclass="code">/etc/wpa_supplicant/wpa_supplicant.conf</span>.
<ahref="https://valh.io/p/raspberry-pi-configure-wlan/wifi--ssh-before-first-boot/">This blog post</a> describes how to set up the initial user and password.
Make sure to set <spanclass="code">dtparam=spi=on</span> in your <spanclass="code">/boot/config.txt</span>, otherwise the SPI connection to the OLED display will not work.
</p>
<p>
All of the following software is written in <ahref="https://www.rust-lang.org/">Rust</a> and available <ahref="https://github.com/FliegendeWurst/raspi-oled/">on GitHub</a>.
<aclass="fancy-name"href="https://github.com/FliegendeWurst/raspi-oled/blob/master/src/bin/take_measurement.rs">take_measurements</a> 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.
<aclass="fancy-name"href="https://github.com/FliegendeWurst/raspi-oled/blob/master/src/bin/refresh_json.rs">refresh_json</a> downloads the latest calendar entries from my <ahref="https://github.com/zadam/trilium/"> Trilium Notes</a> installation and saves them in a JSON file (events.json).
<aclass="fancy-name"href="https://github.com/FliegendeWurst/raspi-oled/blob/master/src/bin/status_check_example.rs">status_check</a> gathers some miscellaneus status information on my other computer systems. The provided example just checks whether GitHub and GitLab are up.
<aclass="fancy-name"href="https://github.com/FliegendeWurst/raspi-oled/blob/master/src/bin/display_all.rs">display_all</a> processes all of this data and displays it on the OLED.
It is run by another Python script (<ahref="https://gist.github.com/FliegendeWurst/087916a7635c2690de609366517a4ae7">turn_on.py</a>) whenever the push switch is pressed.
</p>
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).
For performance reasons (and because the Pi doesn't have enough RAM), I cross-compile these programs using <ahref="https://nixos.org/">Nix</a>.
I'm certain there's a better way to do it, but the following method works for my purposes:
in my <ahref="https://github.com/FliegendeWurst/nur-packages/">NUR packages</a>, run the following command to cross-compile all binaries: <spanclass="code">NIXPKGS_ALLOW_BROKEN=1 nix-build -A raspi-oled-cross</span>.
(Btw, this will first compile LLVM and rustc for some reason. There must be a way to avoid that...)
As always, <spanclass="code">./result</span> is a symlink to the build artifacts.
The binaries (and the libraries they depend on) may be copied to the Pi like this:
