116 lines
6.4 KiB
HTML
116 lines
6.4 KiB
HTML
<!DOCTYPE html>
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<title>Raspberry Pi temperature monitoring + calendar</title>
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<meta charset="utf8">
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<meta name="viewport" content="width=device-width,initial-scale=1"/>
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<link rel="shortcut icon" href="favicon.ico">
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<link rel="stylesheet" href="main.css">
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</head>
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<body>
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<h1 id="header"><img src="favicon.ico" id="favicon"/><a href="/">FliegendeWurst's corner of the WWW</a></h1>
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<h2>Raspberry Pi 0 W + sensors + OLED display = temperature monitoring and calendar</h2>
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<article>
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<img id="hero-img" alt="OLED display showing temperature and upcoming appointments, connected to a Raspberry Pi" src="./raspberry-pi-temperature-monitoring.jpg"/>
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<p>
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With this project, I monitor the temperature and humidity in my room.
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Measurements are taken every 10 minutes and saved in an SQLite database.
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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.
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In the middle, the time until the next appointment, the relative humidity and the current temperature are shown.
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Below that, a graph shows the temperature of the last two days.
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When activating another display mode (not active in the picture), this area of the display instead shows the next seven appointments.
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</p>
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<h3>Hardware</h3>
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<ul>
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<li>computer: <a href="https://www.raspberrypi.com/products/raspberry-pi-zero-w/">Raspberry Pi 0 W</a></li>
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<li>temperature and humidity sensor: AM2302 (wired DHT22)</li>
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<li>OLED display: 1.5 inch SSD1351 (<a href="https://www.waveshare.com/wiki/1.5inch_RGB_OLED_Module">Waveshare</a>)</li>
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<li>yellow push switch: TRU Components PBS-18B 701912</li>
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</ul>
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The components are connected to the Pi as indicated in the wiring diagram below.
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<embed
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type="image/svg+xml"
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src="circuit.svg"
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title="Wiring diagram" />
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<p>
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Wiring of the SSD1351 to the Pi (<a href="https://pinout.xyz/pinout/spi">pinout</a>):
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<ol>
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<li>VCC ↦ 3.3V</li>
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<li>GND ↦ GND</li>
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<li>DIN ↦ SPI0 MOSI (GPIO 10)</li>
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<li>CLK ↦ SPI0 SCLK (GPIO 11)</li>
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<li>CS ↦ SPI0 CE0 (GPIO 8)</li>
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<li>DC ↦ GPIO 25</li>
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<li>RST ↦ GPIO 27</li>
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</ol>
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Wiring of the AM2302 to the RPi:
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<ol>
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<li>VCC ↦ 5V</li>
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<li>second pin ↦ GPIO 26</li>
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<li>third pin ↦ (unused)</li>
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<li>GND ↦ GND</li>
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</ol>
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Wiring of the push switch to the Pi: one pin to GPIO 19, the other to 3.3V.
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</p>
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<h3>Software</h3>
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<p>
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The Raspberry Pi 0 W is running <a href="https://www.raspberrypi.com/software/">Raspberry Pi OS</a> 11 (bullseye).
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It is advisable to enable SSH and configure a WLAN network before booting the RPi for the first time.
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As explained in many tutorials online, this boils down to modifying the SD card image.
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Create an empty file <span class="code">/boot/ssh</span> to enable the SSH daemon, and configure the credentials of your WLAN network in <span class="code">/etc/wpa_supplicant/wpa_supplicant.conf</span>.
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<a href="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.
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Make sure to set <span class="code">dtparam=spi=on</span> in your <span class="code">/boot/config.txt</span>, otherwise the SPI connection to the OLED display will not work.
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</p>
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<p>
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All of the following software is written in <a href="https://www.rust-lang.org/">Rust</a> and available <a href="https://github.com/FliegendeWurst/raspi-oled/">on GitHub</a>.
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<a class="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.
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To mitigate transmission errors, the median of five readings is used.
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The final reading is saved in the specified SQLite database (mine is named sensors.db) along with the current time.
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<a class="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 <a href="https://github.com/zadam/trilium/"> Trilium Notes</a> installation and saves them in a JSON file (events.json).
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<a class="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.
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<a class="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.
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It is run by another Python script (<a href="https://gist.github.com/FliegendeWurst/087916a7635c2690de609366517a4ae7">turn_on.py</a>) whenever the push switch is pressed.
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</p>
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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).
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<pre>
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*/5 * * * * /home/pi/status_check /home/pi/sensors.db 2>/dev/null >/dev/null
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*/10 * * * * /home/pi/take_measurement /home/pi/sensors.db
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15 * * * * /home/pi/refresh_json --no-weekly
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* * * * * /home/pi/display_off off
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</pre>
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<p>
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For performance reasons (and because the Pi doesn't have enough RAM), I cross-compile these programs using <a href="https://nixos.org/">Nix</a>.
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I'm certain there's a better way to do it, but the following method works for my purposes:
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in my <a href="https://github.com/FliegendeWurst/nur-packages/">NUR packages</a>, run the following command to cross-compile all binaries: <span class="code">NIXPKGS_ALLOW_BROKEN=1 nix-build -A raspi-oled-cross</span>.
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(Btw, this will first compile LLVM and rustc for some reason. There must be a way to avoid that...)
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As always, <span class="code">./result</span> is a symlink to the build artifacts.
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The binaries (and the libraries they depend on) may be copied to the Pi like this:
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<pre>
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mkdir /tmp/nixstore
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nix copy --extra-experimental-features nix-command --extra-experimental-features flakes --no-check-sigs --to /tmp/nixstore $(readlink -f result)
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rsync -r --links --info=progress /tmp/nixstore/nix pi@himbeere-null:~/
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</pre>
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On the RPi itself, the binary needs to be adapted to Raspbian and placed in the proper position:
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<pre>
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patchelf --set-interpreter /lib/ld-linux-armhf.so.3 nix/store/*-raspi-oled-*/bin/display_all
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sudo mv nix /
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</pre>
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</p>
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<div class="entry-footer">Posted 5th October 2022</div>
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</article>
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