PoV Wrist Watch (version 2)

A “wrist watch” with a luminous dial, measuring 10cm in diameter.

story

The size of a display on personal electronic devices is a fundamental challenge: there is a trade-off between portability and visibility (screen size). A small device that can have larger display when in use, like a holographic display, would be one of the solution. Unfortunately there is no such product in the market. So I tried to build on my own. As the first PoC of such device formfactor, I made a “wrist watch”.

Overview

PoV wrist watch, showing clock dial PoV wrist watch, stopped PoV wrist watch, with background image
Specifications
DimensionW80xD32xT20[mm]
BatteryLi-Po battery 3.7V 100mAh
MCUESP32S3

Features

mechanical components of the PoV wrist watch
Components
Namedesceription
Li-Po battery 3.7V 100mAhcharged when connected to USB
XIAO ESP32S3control
slip ringsignal and power connection over the spinning joint
bearingmetal ball bearing, ensures smooth rotation and rigdity of the mechanism
infrared photo reflectordetects the rotation position and frequency of the spindle

Algorithm

LEDs must be controlled at the exact timing for PoV display. The program need to know how much the LED bar is rotated at each instant. For this purpose, a photo-reflective marker is attached in the back side of the spindle, and it's sensed by an infrared photo sensor.

photo reflector on the back side of the spindle

The sensor can detect that the bar has passed at a certain angle.

When it's sensed for the second time, the interval of the rotation can be calculated.

The relative angle of the bar from the origin can be computed at every time instance based on the MCU's internal clock, assuming the constant angular speed.

pseudo-code

while True:
  t = current_time()
  if sensor_triggered():
    interval = t - last_t
    last_t = t
  current_angle = (t - last_t) / interval * 360
  (show LED pattern for the current_angle)

山口 辰久