A Very Low-Power LCD Clock
David Johnson-Davies designed a clock that can run for years on a CR2032 battery and indefinitely using a solar cell.
As David Johnson-Davies puts it in his recent Technoblogy blog post, “Although liquid crystal displays (LCDs) are relatively old technology, they still offer several advantages over newer types of display, including low power, low cost, and readability.” With this old-school technology, he’s been able to create a clock display based on an AVR128DA48 processor, which can run for years on a coin-cell battery, or even be powered by a small solar cell.
While standard LCD units – such as the Densitron DG-201208-RP device that he’s using here – don’t consume a lot of power, each segment is driven individually, meaning it requires 28 individual IO pins. Because of this, Johnson-Davies skipped his typical breadboard assembly, and went straight to making a PCB on which to mount the display. The trick is that the segments have to be powered by an AC signal, which sounds difficult, but can be accommodated using software.
Timekeeping is via a RTC peripheral that’s controlled by the onboard 32.768kHz crystal oscillator. In addition to showing the time, it’s also able to use the chips onboard temperature sensor, and can even check its own voltage via the ADC input. The display setup is fairly standardized and should work with a number of other models besides the one he's using. The board also has an I2C connection, allowing it to display info from other sensors as required.