Build Instructions

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• 1

  LED Control Module

  Behaviour

  The LED Control Module (LCM) drives an RGB LED using different colors and blinking patterns indicating the system state.
  

  • Green blinking: Starting. This state is displayed until Volumio is completely operational.
  • Green steady: Started. Normal operation.
  • Red blinking: Stopping.
  • Red slow blinking: Stopped. Indicates that the system can be powered off. Turning off the Pi during normal operation is not recommended, and it cannot be powered off completely using software.

  
  The state diagram (done with yEd) shows the different state transitions.

  Implementation

  The LCM is implemented using an ATTiny85 microcontroller listening to level transitions on the a GPIO pin on the Pi, and driving an RGB LED according to the above state diagram.

  The GPIO pin on the Pi is controlled by regular startup and shutdown scripts that have to be installed after installing Volumio. The pinout is described in the Prototype step.

  Hardware

  The circuit (drawn using EasyEDA) consists of the following main components.

  • A six-way AVR ISP header (J1).
  • Input header, including the GPIO connection from the Pi (H2).
  • Output header, for connecting the LEDs (H1).
    • The LEDs will only work when pins 1 and 2 of JP1 are shorted. Otherwise pins 1 and 2 of H1 can be used for an alternative connection. Used for debugging.
  • The resistors R1 and R2 are used to reduce the basic brightness of the LEDS.
    • Potmeters R3 and R4 can be used to further tune the brightness.

  The hardware components (not yet mounted in the picture) are mounted using through hole soldering on a simple PCB.

  Software

  The LCM firmware can be found on GitLab. The code is fully documented. From a high level perspective, it implements the state machine depicted above and uses (low frequency) PWM to blink the LEDs according to the current state.

  The software was developed on a Mac using the GNU AVR toolchain. Conveniently, the toolchain comes installed with the Arduino IDE. 

  The Arduino was used as a programmer, with the ATTiny being the target, connected through the P1 header on the PCB.

• 2

  Install Volumio and scripts

  NOTE: some familiarity with Volumio is assumed. If you are not familiar, their Quick Start User Guide may be helpful.

  Install Volumio

  Download Volumio, and install it on a micro SD Card. A minimum of a 4GB is required, 8 GB is recommended. 

  Check the Raspberry Pi Foundation's website for some useful guides on how to write an image file onto an SD card using your favourite OS.

  Install Scripts

  Boot into Volumio, and make sure you can access it over SSH.

  Follow the instructions from the GitLab project. The GPIO pin can be changed in the startup script.

• 3

  Prototype

  Now it's time to put together a prototype, to see if the hardware and software.

  The diagram (created using Fritzing) shows the system as it becomes one step closer to the real hardware.

  And now with the real hardware (spot the differences...):

  On the Pi, GPIO 17 was chosen as the LCM GPIO-in (eventually connected to INT0 on the ATTiny) because this pin goes to low when Pi is shut down. It can be changed to another pin as long as it behaves as GPIO 17 and does not conflict with the pins specified by HiFiBerry.

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