Mechanical Design

The whole display is simply an bunch of character modules joined together.

The motor is attached to a simple gear system in order to centre the output shaft and minimise the size of the character drum. Electonics can be slid into the module enclosure from any side, and a small plastic tab holds the hall effect sensor in place. M3 threaded shaft runs through all the modules and clamps the modules together. Endplates are mounted with 2 screws each and give the display a clean look, hiding all the screws. The character drum has a magnet placed at a specific location, which when detected by the hall effect sensor, allows us to accuractly control the position of the motor.

Electronics

Each module consists of:

• 1x 28BYJ-48 Motor
• 1x ULN2003 Motor Driver
• 1x A3144 Hall Effect Sensor
• 1x PCF8575 I2C GPIO Expander

The leftmost module, or "master" module, also contains

• 1x ESP32 C3
• 1x USB C Power Connector

The master module communicates with all the other module using an I2C bus. Each module has a different address on the bus, which is pre-set by soldering contacts on the back of the PCF8575.

Wiring

Wiring is relatively straightforward, but becomes more complex when you consider the small volume we need to pack everything into.