Lightbar

Value tweening for PWM pins on Raspberry Pi, using Pi-Blaster, with systemd services to fade in & out at boot & power off, respectively.

When daemonized, utilizes D-Bus to provide instant value changes, avoiding Ruby startup times on lower end Raspberry Pi models.

Created to control an LED light strip connected a Raspberry Pi Zero W interfacing via an admittedly primitive 3.3v to 12v inverting level shifter.

Installation

Note that gem install and make install methods might require sudo. These are written from the perspective of a root user.

Manual

$ git clone https://github.com/RyanScottLewis/lightbar-rb.git
$ cd lightbar-rb
$ make
$ gem install pkg/lightbar-x-x-x.gem
$ make install

Pacman

$ git clone https://github.com/RyanScottLewis/lightbar-pacman.git
$ cd lightbar-pacman
$ makepkg -i

Post-Install

Systemd Units

Enable both the lightbar and the ability to fade in on startup & fade out on shutdown:

$ systemctl enable --now lightbar lightbar-fade

D-Bus

D-Bus is required when running as a daemon with the --daemon option and is run on the system bus.

A D-Bus policy is installed which allows the root user to create the D-Bus address on the system bus as well as allows any user of the lightbar group to send messages to it.

So, if you'd like a user other than root to control the lightbar, simply add them to the lightbar group.

If you intend to add a user to the lightbar group have them control the lightbar within the same session, you need to reload the D-Bus configuration.
You can do this by running: systemctl reload dbus

Pi-Blaster Configuration

Pi-Blaster is required for use with Lightbar. The lightbar Systemd service requires Pi-Blaster as a dependency. But, if you are not using Systemd, then be sure Pi-Blaster is running first.

Be sure the pin you are using for PWM is enabled to be used by Pi-Blaster. This can be set with the --gpio option when running Pi-Blaster or if using it's Systemd service, creating the /etc/default/pi-blaster file and adding --gpio to the DAEMON_OPTS variable:

DAEMON_OPTS="--gpio 4,17,18,27,21,22,23,24,25"

Usage

Simple

Tween PWM output value from 0 to 1 over 5 seconds through Pi-Blaster on BCM pin 18:

$ lightbar --from 0 --to 1 --duration 5 --pin 18

Daemon

Run as daemon (to avoid Ruby startup times on slower systems):

$ lightbar --daemon

Then in another terminal (or same one if you CTRL+Z then run bg or run with a trailing &)

$ lightbar --from 0 --to 1 --duration 5

This causes the application to communicate with the daemon over the system D-Bus.

However, this still includes the same Ruby startup times, defeating the purpose of daemonizing.
To get past that startup time hurdle, we ship with a shell script which simply calls the tweening function over D-Bus directly, bypassing having to use Ruby to cause the tweening whatsoever.

For example, the above example is equivalent to the following:

$ dbus-send --system --print-reply --type=method_call --dest=org.Lightbar / org.Lightbar.tween double:0 double:1 double:5

For convienence, we profile the lightbar-tween script that provides the same functionality:

$ lightbar-tween 0 1 5

Level Shifter

To provide the correct amount of power to the LED strip, we utilize a level shifter to convert from the Pi's 3.3vdc PWM output to what the LED strip requires, which is 12vdc in this case.

                            +12v
                              |
Raspberry Pi   170 Ohm    Collector
  PWM Pin    - Resistor - Base (NPN)
  (BCM 18)                Emitter
                              |
                              0v

Justification

This was originally written in Crystal, which worked very well. There was no startup times.
However, building the application required cross-compiling to the ARMv6 platform (for Pi Zero W).
This required maintaining a cross-compilation platform on the Pi which needed a Crystal re-compilation after each Crystal version was pushed. This takes a long time on my poor little Zero W, where as a single Ruby install is finished in no time and widely available. This could have easily been written in Python, Lua, or any other widely available interpreted language. I just prefer Ruby.