pwcazenave/light-sensor-api

Light sensor API

Hardware

I've written this with two light sensor configurations in mind:

1. https://pimylifeup.com/raspberry-pi-light-sensor/
2. https://tutorials-raspberrypi.com/photoresistor-brightness-light-sensor-with-raspberry-pi/ (and https://tutorials-raspberrypi.com/mcp3008-read-out-analog-signals-on-the-raspberry-pi/)

The former uses the photoresistor to time how long it takes to charge a capacitor with the light resistor in the circuit. The latter converts the light resistor values from analog to digital.

Installation

These instructions assume a raspos host and a raspberry pi (any generation).

Enable SPI

1. sudo raspi-config
2. Interface options > I4 SPI > Enabled
3. Reboot

Quick start

git clone git@github.com:pwcazenave/light-sensor-api
cd light-sensor-api
apt install python3-venv
python3 -m venv venv --system-site-packages
. venv/bin/activate
pip3 install -r requirements.txt
# Open up the firewall
sudo ufw allow 8000/tcp
cd app
gunicorn --workers=1 --bind 0.0.0.0:8000 main:app

podman and systemd

Note: for this to work, we need podman v3.2+, which isn't yet available for Debian 11/raspos. Hence, we have to do a hacky podman install

podman install

sudo apt install -y podman  # install podman so it sets up subuid/subgid for us
wget https://github.com/mgoltzsche/podman-static/releases/download/v4.0.2/podman-linux-arm64.tar.gz
tar -xvf podman-linux-arm64.tar.gz
sudo cp -r podman-linux-arm64/{etc,usr} /
reboot

Now we have podman v3.4+ running, we can set up the light container with systemd.

git clone git@github.com:pwcazenave/light-sensor-api
cd light-sensor-api
sudo usermod -G spi $USER
./make_image.sh
sed 's/LOCAL_USER/'$USER'/g' systemd/podman-light.service | sudo tee /etc/systemd/system/podman-light.service
sudo chmod 644 /etc/systemd/system/podman-light.service
podman stop light
podman rm light
sudo systemctl daemon-reload
sudo systemctl enable --now podman-light.service
sudo ufw allow 8000/tcp

venv and systemd

As root:

cd /opt
git clone git@github.com:pwcazenave/light-sensor-api
mv light-sensor-api light
cp /opt/light/systemd/light.service /etc/systemd/system/light.service
chmod 644 /etc/systemd/system/light.service
cd light
apt install python3-venv
python3 -m venv venv --system-site-packages
. venv/bin/activate
pip3 install -r requirements.txt
ufw allow 8000/tcp
systemctl daemon-reload
systemctl enable --now light.service

Usage

Simplest way to use the API is to just curl the relevant endpoints, which are:

Timed

The lightness is measured here as number of iterations needed to go from low to high on the resistor. Hence, a lower value is a higher light intensity (brighter).

• /api/v1/timed/current
• /api/v1/timed/last/<period>, where <period> is some integer number of seconds

Analog to digital

This uses the MCP3008 to convert the analog signal to a digital one. Here, a low value is dark, high is light.

• /api/v1/analog/current
• /api/v1/analog/last/<period>, where <period> is some integer number of seconds

For example to fetch the current instantaneous analog value:

curl http://example.com:8000/api/v1/analog/current | jq .current

To average over 60 seconds:

curl http://example.com:8000/api/v1/analog/last/60 | jq