AncientJames/Scanwheel

Source: https://github.com/AncientJames/Scanwheel

Scanwheel

Scanwheel is a drum style mechanical television you can build yourself.

It features five windows side by side, each taking advantage of the same scanning drum, but with separate LEDs to allow separate images. The windows are a luxurious 9mm x 8mm each, with 20 scan lines.

The software uses the Pico’s PIO hardware to drive the LEDs, allowing extremely high horizontal resolution, so the image quality is a lot better than the 20p line count would imply.

There’s an accompanying video showing the build process, with footage of it in action.

Parts

You’ll need the following components:

Symbol Technologies 21-02485 stepper motor Old but still widely available online. It has an unusual wiring setup, but it can be used as a bipolar stepper motor by cutting the white wire before it goes into the 4 pin connector as described here.

A4988 based StepStick Something like the Pololu 1182. There’s a potentiometer to adjust the current limit. Turn it down as low as it will go before the motor stops turning - it will be almost at the bottom end of the pot’s range.

100uF Capacitor Across motor power & ground.

Raspberry Pi Pico Doesn’t matter which, but use a Pico W if you want to stream wirelessly to the device.

LEDs In the design presented here there’s a single RGB LED and 4 white LEDs, which seemed like a good compromise between wiring complexity and image quality.

Resistors Each LED needs a current-limiting resistor. The R, G and B channels need to be matched in brightness to each other, and the overall RGB brightness needs to be matched to the white LEDs. All of them together must draw no more than 50mA from the Pico’s GPIO pins.

This means you need to determine the resistor values for the specific LEDs you have sourced - measure the actual current drawn with your LED and resistor connected.

I found that the red channel of the RGB LED was the deciding factor, at 10.75mA with a 120R resistor. Matching that brightness required 260R for green (2.9mA), 220R for blue (2.8mA) and 82R for white (4.6mA)

Printing

The 3D models are in the parts directory. There are three printed parts; the base, the drum and a lid.

They will fit best if they are scaled to compensate for shrinkage during printing - on my printer, using PLA, I find that a scale of 100.33% in X and Y and 100% in Z achieves this.

The base and the lid don’t need any special treatment, but the drum requires a bit of care in printing. Use the highest quality settings - small layer height and low speeds - because small flaws in the pinholes are very visible in the final display.

The sides of the drum are a single wall thick. In Orca Slicer this means you need to set ‘Wall Generator’ to ‘Arachne’. The infill should be 100%.

The OpenSCAD model for the drum is included, allowing you modify the design. One of the parameters is nozzle diameter, and if your printer supports a 0.2mm nozzle I recommend using that variant. You might find you need to fit a washer over the drum’s spindle to increase its moment of inertia.

Wiring

Software

The software runs on Micropython 1.28. The device code is in the pico directory - scanwheel.py is the driver, along side some example apps.

The code in host runs on the PC - videostream.py streams videos over a network connection to streaming.py on the device. pngdirect.py converts image files to the raw framebuffer format used by scanwheel.