Josh EJ's "Date Clock" Tracks a Whole Year — and Has a 50-Year Battery Life

Maker Josh EJ has upcycled an analog clock into a calendar — using an underclocked Arduino Pro Mini board to deliver a claimed 50-year runtime from a single AA battery.

"The idea for this project is to repurpose an analog wall clock to display the date instead of the time. The hour hand will display the month (1 to 12) and the minute hand will correspond to the day," EJ explains. "The Calendar Clock visually indicates how much of the year has passed by and can potentially incentivize people to plan their days and use time more effectively."

The heart of the build is, of course, a standard analog quartz wall clock, powered by one AA battery. To this, EJ has added an Arduino Pro Mini microcontroller board with an external real-time clock module — replacing the signals from the quartz crystal which would normally see the clock ticking once per second, extending the time it takes so that one revolution of the hour hand will take an entire year.

A simple paper cover for the clock face lets you read the day of the month using the minute hand — with the Arduino coded to correctly advance the hand for months shorter than the full 31 days of a complete rotation — but it's the power management that makes the project stand out. The microcontroller is underclocked from its default 16MHz to just 1MHz, which allows it to run from the clock's 1.5V battery — while having the handy side-effect of also dropping its output logic to a clock-friendly 1.5V.

Coupled with the removal of the board's voltage regulator and LEDs and a low-power sleep between "ticks," the clock can run for some considerable time — an estimated 55 years, if fitted with a 3Ah battery. "Although the current draw is significantly low, alkaline batteries have a shelf life of 10-15 years," EJ admits. "It would be interesting to see the real-world battery life but would have to wait for a long time though."

The full project write-up, including source code and wiring instructions, is available on EJ's Instructables page.