temperature-sensor

This code can read out a BME280 temperature sensor, store the measurements in a database and display them on an LCD screen. To do this, I had to get my Raspberry Pi ready, and for the sake of completenesss I'll add all the steps, so I can easily replicate it on another Raspberry Pi.

General Raspberry Pi setup stuff

1. Download Raspbian and create a bootable SD card.
2. In the /boot partition, add an empty file called ssh to get the Pi to use SSH, so you don't have to connect a monitor to it to get into it.
3. In the /boot partition, add a file called wpa_supplicant.conf with the below contents, with the configuration of my WiFi access point. This will make the Pi connect to WiFi automatically by placing the configuration file at the right path.
4. Create a user called toon for myself, and create a sudoers file at /etc/sudoers.d/010_toon-nopasswd with the contents toon ALL=(ALL) NOPASSWD: ALL. Verify that sudo still works after that.
5. Destroy the password for pi by editing /etc/shadow.

The contents of /boot/wpa_supplicant.conf:

country=NL
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
network={
    ssid="MY_WIFI_SSID"
    psk="MY_WIFI_SSID_PASSPHRASE"
    key_mgmt=WPA-PSK
}

Connecting the sensor

The sensor package this script was built for, namely the Velleman VMA335, uses I2C address 0x76 and port 1. With the systemd unit you should be able to set an environment variable to modify the script's behavior but I have not tried that yet.

Find Raspberry Pi pinout diagrams at pinout.xyz. Connect the pins as follows:

• Connect a 3.3 volt pin (do not use a higher voltage!) on the Pi, to the VCC pin on the sensor.
• Connect a ground pin on the Pi, to the GND pin on the sensor.
• Connect the SDA pin on the Pi to the SDA pin on the sensor.
• Connect the SDC pin on the Pi to the SDC pin on the sensor.

Connecting the LCD screen

The LCD screen operates in 4 bit mode, and the one I use is backlit, so it will need 12 of its pins connected to the Raspberry Pi.

The following pins need to be connected:

Pin	Connection
1	Ground
2	5V
3	See below
4	Board 22, BCM 25
5	Ground
6	Board 18, BCM 24
7	do not connect
8	do not connect
9	do not connect
10	do not connect
11	Board 16, BCM 23
12	Board 11, BCM 17
13	Board 12, BCM 18
14	Board 15, BCM 22
15	5V
16	Ground

You need to connect both a 1kOhm resistor and a 2.2kOhm resistor to pin 3. The 1kOhm resistor needs to be connected to 5V and the 2.2kOhm one to ground.

Python setup:

install.sh will do the following:

1. sudo modprobe i2c_dev
2. Check for the correct Python packages, and say which apt-get install commands you need to run if you don't have them
3. Install the necessary scripts and set up a venv at /opt/sensor-data
4. Install systemd units and explain how to use them.

Data will end up in the SQLite database /var/lib/sensor-data/sensor-data.db.

Enabling and starting the systemd services should take care of everything. There are three units that are installed by the install script:

• sensor-data.service collects the sensor data from the sensor and stores it in the database;
• lcd-screen.service polls the database and displays the last five seconds' worth of data on the LCD screen connected to the Raspberry Pi;
• sensor-data-webview.service serves a web site on port 8000 which displays a graph of the temperature data.

Database and sensor data notes

The SQLite backend I've made uses FLOATs as columns for the sensor data. The reason is pretty unelegant: I wanted to use DECIMALs but it turns out SQLite would have just made FLOATs out of those.

The sensor returns float data because it can measure fractional quantities. According to Bosch's data sheet of the BME280 sensor, the tolerances are:

• Temperature: +/- 1.0 degrees Celcius between 0 and 65 degrees Celcius,
• Humidity: +/- 3% relative humidity,
• Pressure: +/- 1.0 HPa between 0 and 65 degrees Celcius.

Anecdotally, however, the sensor seems to be accurate to well within the following tolerances:

• Temperature: +/- 0.1 degrees Celcius between 0 and 65 degrees Celcius,
• Humidity: +/- 0.5% relative humidity,
• Pressure: +/- 0.5 HPa between 0 and 65 degrees Celcius.

Sample nginx configuration

The below snippet is how I configured nginx to act as a reverse proxy for the web view on my personal Pi at home.

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    server_name _;

    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl default_server;
    listen [::]:443 ssl default_server;

    server_name _;

    # Note: You should disable gzip for SSL traffic.
    # See: https://bugs.debian.org/773332
    # Read up on ssl_ciphers to ensure a secure configuration.
    # See: https://bugs.debian.org/765782

    # Self signed certs generated by the ssl-cert package
    # Don't use them in a production server!
    ssl_certificate /etc/ssl/certs/ssl-cert-snakeoil.pem;
    ssl_certificate_key /etc/ssl/private/ssl-cert-snakeoil.key;

    location / {
        proxy_pass http://127.0.0.1:8000;
    }
}