“This article describes a minimalist ATtiny85-based watch using 12 APA102 RGB LEDs, arranged like a clock face, to show the time analogue-style. It shows the time to the nearest second, and the positions of the hour, minute, and second hands are displayed in blue, red, and green respectively. Intermediate times are shown by the relative brightness of two adjacent LEDs:
To show the time you press the button on the watch face; the display is blanked after 15 seconds to save the battery. The ATtiny85 is clocked by a crystal, ensuring that the time is accurate to within a few seconds a month.
Introduction
This is the third of my LED-based watch designs. My earlier Tiny Time 2 Watch used monochrome LEDs, and distinguished between the hour and minute hands by flashing the minutes LED. I thought I had taken the Tiny Time watch concept as far as it could go until I read about the APA102 LEDs, full colour LEDs in a 2020 package that cost under $5 for a dozen. You can control them using a two-wire serial interface, and daisy-chain a virtually unlimited number of them together, with individual control over the colour and brightness of each LED.
I realised that this would allow me to design an ATtiny85-based watch, using two I/O lines to control the 12 RGB LEDs, two I/O lines to connect to an external crystal, and one I/O line free to connect to a pushbutton to display the time. Because the timing is performed by the ATtiny85 there’s no need for an RTC chip.”