Simplest Attiny85 based watch

We are used to see different kinds of DIY watches. Some of them try to stuff lots of functionality, other make fancy time telling format or simply mimic smart watches. But why not build a watch that only tells time in a simple manner.

This Attiny85 watch is what it does. Schematic is very simple and straight forward – Attiny85 microcontroller, 12 LEDs, push button, three resistors and coin cell battery. Such simplicity allows building small watch shaped PCB where all components are fit to look nice and light. Since microcontroller comes with only 5 I/O pins (excluding reset), LEDs are connected using smart Charlieplexing method that allows saving pins for simple I/O tasks. The only shortcoming of this watch that time is counted using microcontrollers internal clock which may not be very accurate. As author states, clock is capable of keeping time within two minutes over 24 hour period. There are ways of calibrating internal clock source by manipulating internal registers, but in the end there is no point, as oscillator is dependent on many external effects like temperature, supply voltage and other factors. If you look at PCB, you can obviously see that there is still enough space to squeeze in RTC or external oscillator. But these are the ideas for another project…

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