Build Your Own Scanwheel: A DIY Mechanical Television

The Scanwheel is an enthusiast project that allows you to construct a drum-style mechanical television from scratch. Unlike modern screens, this device uses a physical rotating drum to create images.

📺 System Overview

The device utilizes a single scanning drum to drive five side-by-side windows. While they share the same drum, each window is powered by its own dedicated LEDs, enabling the display of separate images simultaneously.

Technical Specifications

Window Dimensions: A luxurious $9\text{mm} \times 8\text{mm}$ per window.
Vertical Resolution: 20 scan lines.
Horizontal Precision: By leveraging the PIO (Programmable I/O) hardware of the Raspberry Pi Pico, the system achieves an extremely high horizontal resolution. This ensures the visual quality far exceeds what one would expect from a mere 20p resolution.

🛠️ Hardware Requirements

1. The Motor

You will need a ~~modern~~ Symbol Technologies 21-02485 stepper motor. Although it is an older model, it remains widely available through online marketplaces.

Wiring Note: This motor has a non-standard wiring configuration. To operate it as a bipolar stepper motor, you must cut the white wire before it enters the 4-pin connector.

Pro Tip: When adjusting the potentiometer, turn it down as far as possible without stopping the motor's rotation; the sweet spot is usually at the very bottom of the range.

2. The Controller

A Raspberry Pi Pico is required.

Standard Pico: Works fine for basic operation.
Pico W: Recommended if you wish to stream content wirelessly.

3. The Lighting (LEDs)

The build uses a combination of one RGB LED and four white LEDs. This setup balances wiring simplicity with acceptable image quality.

Crucial Constraints:

The R, G, and B channels must be brightness-matched.
The combined RGB output must match the intensity of the white LEDs.
Total current draw must not exceed 50mA from the Pico's GPIO pins.

To calculate the necessary resistors, use Ohm's Law: $I = \frac{V_{source} - V_{forward}}{R}$

The following table outlines the specific values used by the creator to achieve balanced brightness:

LED Channel	Resistor Value	Current Draw
Red	`120R`	$10.75\text{mA}$
Green	`260R`	$2.9\text{mA}$
Blue	`220R`	$2.8\text{mA}$
White	`82R`	$4.6\text{mA}$

🖨️ 3D Printing Guide

The necessary models are located in the /parts directory.

Printing Checklist

Scale X and Y axes to 100.33% (to compensate for PLA shrinkage).
Keep Z axis at 100%.
Set 'Wall Generator' to Arachne (if using Orca Slicer).
Use a 0.2mm nozzle for the drum (if supported).

Drum Fabrication

The drum is the most critical component. Because pinhole imperfections are highly visible, you must use high-quality settings:

Low printing speeds
Small layer heights

The project includes an OpenSCAD model for the drum, allowing you to tweak parameters. Additionally, you may want to add a washer to the drum's spindle to increase its moment of inertia.

💻 Software Architecture

The software is split between the host computer and the microcontroller.

File Breakdown

/pico directory:
- scanwheel.py: The primary driver for the hardware.
- Various example applications.
/host directory:
- videostream.py: Handles the network stream from the PC.
- streaming.py: The receiving end of the stream on the Pico.
- pngdirect.py: A utility to convert standard images into the raw framebuffer format required by the scanwheel.

# Example logic for framebuffer conversion
def convert_to_framebuffer(image_path):
    # Logic to transform PNG to raw scanwheel format
    pass