Introduction

I was inspired to design this watch after reading about Sam DeRose's Nerd Watch which takes minimalism one step further. It seems fortuitous that you can drive 12 LEDs from four I/O lines using charlieplexing, exactly the number you need to display a clock face, taking advantage of the fact that you can drive two LEDs from every different pair of the I/O lines. One more I/O line is needed to detect the button press, neatly using the five I/O lines available on the ATtiny85.

To show the time you press the button on the watch face, and the time is then displayed for four seconds. It lights one LED to show the hour, and flashes another LED to show the minutes to the nearest five minutes, like the hour and minute hands on a clock. If only one LED lights up you know that both hands are pointing to the same hour mark.

The Tiny Time watch uses the ATtiny85's built-in oscillator, fine-tuned to get it as accurate as possible, and my watch keeps time to within a couple of minutes over 24 hours. You can adjust the time by holding down the button for more than four seconds.

It's powered by a CR2032 button cell, and I've used several techniques to reduce the current consumption, so the battery should last for over a month. The total component cost, excluding the PCB, is about £5/$5, and it's straightforward to build.