Deposition

This repository contains Deposition (short for Desk Position), a small embedded application that controls the position of an height-adjustable desk. This is the application in action:

demo.mp4

Video of height-adjustable desk moving up and down

How it works

The micro controller gets inputs via 4 buttons: up, down, position 1, position 2. When one of the buttons is pressed, the desk starts moving. Required hardware:

• 1x precision potentiometer Bourns 3590, supports 10 full rotations
• 1x retractable lanyard/key chain
• 1x OLED 128x64 dot matrix SSD1306
• 2x photo coupler EL817
• 4x button
• resistors
• cables
• enclosures, 3D printed
• micro controller dev board ESP32-WROOM-32

Position Detection

test-bench.mp4

Video of thread begin pulled, hooked onto screws and released, display shows current distance

The current height is detected with the micro controller's builtin ADC which is connected to a potentiometer. This potentiometer is in turn mechanically connected to the insides of a retractable lanyard with a few 3D printed connectors. The thread of the lanyard is connected to a horizontal stabilizer bar underneath the desk. When the height increases, the thread is pulled further out and when it is reduced, the spiral torsion (=clock) spring of the lanyard retracts the thread. This in turn rotates the potentiometer clockwise or counterclockwise. This is how it looks inside:

Further details can be found in resources/height-adjustable-desk.pdf.

Control of the desk motors

The desk is moved by motors located in the desk's legs. The were pre-installed in the commercial product and could be controlled with two buttons: up and down. However, the control panel did not support moving to a memorized position. For Deposition, the buttons were replaced with photo couplers to emulate their open/closed behavior while keeping the two circuits electrically separated.

The circuit diagram can be found in resources/height-adjustable-desk.pdf.

Enclosure

The enclosures were designed in FreeCAD and later on in OpenSCAD. They are all 3D printed. All models can be found under <./cad>.

Lessons learned

• General 3D design with FreeCAD and OpenSCAD was not hard after getting used to the general design process.
• Rust embedded ecosystem is nice: GPIO pins cannot be used multiple times by accident because you take ownership.
• Twisting wires is a bad idea if they are not a differential pair. I could not even get buttons or display (with 10Hz I2C) to work this way.

Development environment

In addition to normal Rust, install the ESP32 Rust toolchain via espup. Remember to always source ~/export-esp.sh. You can then use the normal cargo commands, e.g. cargo run --release¹ to compile and flash the program onto the micro controller if it is connected.

For testing, Wokwi is available. Unfortunately, the committed test does not work because it fails to read from the persistent memory. The test can be run with wokwi-cli --scenario wokwi-tests/go_to_options.test.yaml if Wokwi is installed in v0.14.0 (and possibly others). Wokwi also requires the environment variables WOKWI_CLI_TOKEN and GITHUB_TOKEN to be set.

Footnotes

1. Debug mode will likely not working due to timing-sensitive peripherals. ↩