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auto-brightness-rpi-tsl2591

Set monitor brightness automatically to match ambient light with Raspberry Pi and TSL2591 lux sensor.

What?

I wrote this code for a DIY digital photo frame project using Raspberry Pi connected to a monitor. Based on the TSL2591 lux sensor readings of visible ambient light, the app sets matching monitor brightness.

How Does It Work?

The code maps readings of TSL2591 sensor to detected monitor's brightness scale. Underneath it uses ddcutil command to set the monitor's brightness setting.

Ambient light is sampled once per second. Even though it's possible to achieve stable lighting conditions with artificial light, a digital photo frame most likely sits somewhere in your room where daylight can reach it. Daylight fluctuates constantly, even if very slightly. Because of that the app does not immediately set brightness to currently detected value to avoid constant flickering. Instead brightness is set to the average of the readings from last 10 seconds. In relatively stable lighting conditions this reduces the annoying flickering effect. When lighting changes significantly (e.g. when you switch light on after dark), it takes around 10 seconds to gradually reach the target value. I haven't tested the app with strobo lights though.

Currently, the entire range of sensor's readings is mapped linearly to monitor's brightness setting scale (usually 0-100). This approach is rather naive, but seems to work well enough for the monitor I'm using. I might think of implementing some kind of enveloping for the values at some point though.

Raspberry Pi Setup

0. Attaching TSL2591 to Raspberry Pi

See the guide about wiring the sensor to Raspberry Pi.

1. Install dependencies

The assumption is you're using Raspberry Pi OS or another Debian based distro on your Pi.

Update the system and install the OS packages needed

sudo apt update && \
sudo apt upgrade -y && \
sudo apt install -y \
    python3-dev \
    python3-venv \
    ddcutil \
    git

python3-dev and python3-venv are needed to build the source code into a stand-alone executable. ddcutil is a command line tool to set monitor's brightness. Git is optional, and only needed if you'll be cloning the repository.

32-bit System

There are some extra steps needed when setting up a 32-bit system. You can check if your system is 32 or 64-bit with getconf LONG_BIT command.

You need to install an extra package to make numpy (one of the apps dependencies) work:

sudo apt install libatlas-base-dev

2. Build and Test

Clone the repository using git glone, or download the source code zip and extract e.g. to /home/pi/auto-brightness-rpi-tsl2591.

cd into the directory and run

./build_pex.sh

32-bit System

You need to prepend the ./build_pex.sh with CFLAGS=-fcommon:

CFLAGS=-fcommon ./build_pex

Wait for the build to finish. A new file auto-brightness-rpi-tsl2591.pex has been created.

Display help message:

./auto-brightness-rpi-tsl2591.pex --help

Test the app with:

./auto-brightness-rpi-tsl2591.pex --dry-run --verbose

By default, monitor's brightness VCP feature code is 10. Run ddcutil capabilities to find out if it's the same on your monitor, e.g. this is part of the output of ddcutil capabilities command:

...
VCP Features:
   Feature: 02 (New control value)
   Feature: 04 (Restore factory defaults)
   Feature: 05 (Restore factory brightness/contrast defaults)
   Feature: 08 (Restore color defaults)
   Feature: 09 (Brightness)
   Feature: 12 (Contrast)
...

In this case, we can see that the brightness feature code is 09. Add --vcp option when executing:

./auto-brightness-rpi-tsl2591.pex --dry-run --verbose --vcp 09

If everything works fine you should see the output such as this, with new value being printed every second:

[2023-02-01 11:46:40] INFO Initial brightness: 84
[2023-02-01 11:46:40] INFO Visible light: 1711512060    Brightness: 80
[2023-02-01 11:46:41] INFO Visible light: 1706531400    Brightness: 79

Run

If testing went well, it's time to make the app actually change the monitor brightness (remember to also set --vcp option if feature code is different than 10):

./auto-brightness-rpi-tsl2591.pex --verbose

Fiddling With Sensor Settings

There's a plethora of different monitors out there and their actual brightness settings differ greatly. If the brightness set by the app does not suit your taste, you may try to modify the code in main.py file, particularly the two lines setting gain and integration time of the TSL2591 sensor:

   sensor.gain = adafruit_tsl2591.GAIN_HIGH
   sensor.integration_time = adafruit_tsl2591.INTEGRATIONTIME_300MS

See the description of the possible values for details.

The values set in the code are corresponding to settings I found working best with my particular monitor. I'm planning to add a command line option for setting these in the future, so it's not necessary to edit the code. For now, you can change the code in main.py and re-build the app (./build_pex.sh).

[Optional] Auto-Start

To start the brightness control automatically when Pi gets booted, enable auto-login using raspi-config and add the following line to /etc/rc.local just before exit 0 (change path to match the location of your file, also if your Pi username is different than pi, change that as well):

su pi -c "/home/pi/auto-brightness-rpi-tsl2591/auto-brightness-rpi-tsl2591.pex &"