momentum-v1

An elegant circuit board for a more civilized age

Star Wars: Revenge of the Ditch
26 May 2023

Description

This is a simple prop controller designed for a Ditch Day stack at Caltech. Ditch Day is a tradition where undergraduate seniors design elaborate themed experiences, called "stacks", for the enjoyment of younger students on a date kept secret until the last minute.

This Ditch Day stack featured custom lightsabers with WS2812 LED lighting and motion-reactive sound effects. This repository contains the hardware and firmware used in the lightsabers.

Hardware

• Raspberry Pi Pico
• MT-3608 boost converter module, permitting flexible power input (designed for 2S D cell batteries)
• 1x WS2812 LED output with 5V buffer
• PAM8302 2.5W Class-D audio amplifier
• 2x pushbutton input with RC debouncing
• Socket for MPU-6050 IMU

Fabrication output is generated using KiKit, targeting the JLCPCB assembly service. It is recommended to order panels to lower costs; 10 panels of 4 were fabricated for Ditch Day with a target of 20 production units.

• Schematic
• Interactive BOM

Firmware

The code is written using the Raspberry Pi Pico C SDK and has been tested to build and run properly on Ubuntu 22.04 LTS (with the Pico SDK installed per Raspberry Pi's instructions) and Windows (using the Windows installer).

Assembly Notes

• The MPU-6050 is intended to fit over the MT-3608 boost converter. Header pins can be soldered to the MPU-6050, then soldered into the row of pins on the PCB. Take care to respect the orientation.
• The LED output pads are not in the same order as a typical WS2812 strip (board errata).
• There is no reverse polarity protection, though the MT-3608 may be tolerant of reverse polarity.

Usage

uf2 binaries as used for Ditch Day are provided in firmware/bin/. The nomenclature 44k1r2 refers to audio at 44.1 kHz sampling rate and with every sample repeated twice (so PWM clock frequency of 44.1 kHz * 2 * 256).

To modify the code,

1. Install the Pico C SDK, including pico-extras. You might also install Visual Studio Code and the relevant extensions (CMake, C/C++, etc.) if you prefer an IDE.
2. Modify definitions in config.h as appropriate for e.g. flash-on-clash animations
3. From firmware/src/, mkdir build
4. cd build
5. cmake ..
6. make (or ninja if using the Windows SDK installer)

Upload the code as with any RP2040 binary using the USB bootloader (plug in USB while holding down BOOTSEL on the Pico, and drag-and-drop the .uf2 file into the RPI-P2 'flash drive' that appears.

Customizing sounds

A converter utility Python script is provided, which takes a set of WAV files (one poweron.wav 'ignition' sound, one poweroff.wav deactivation sound, one 'hum.wav' idle sound, and any number of clash0.wav clash1.wav... clash sounds and swing0.wav swing1.wav... swing sounds) and creates a C header file with uint8_t arrays and suitable definitions.

If you would like to use your own sounds, convert them to WAV, run the Python script on them, and include the generated .h file in speaker.c.

Future Improvements

• Single custom PCB with more flash (RP2040 supports up to 16 MB versus Pico stock 2 MB)
• SD card support
• Motion detection uses interrupt features of MPU-6050, which use only the accelerometer. Clash and swing sensing can be improved.
• Due to budget, there is no physical power switch, and WS2812s have high quiescent current. Add a physical power switch or a MOSFET.
• Design for Li-Ion power versus 2S D-cell (chosen for safety)

Go to bed frosh, Ditch Day is Tomorrow!