Big Time: A Large 7-Segment Display Clock
David Johnson-Davies' giant digital clock uses a few tricks for an elegant electronics solution.
7-segment displays have been staples in hackerdom for decades, and while sometimes you need something small, at other times a very large display is in order. In the case of David Johnson-Davies' Big Time project, he used four 56.80mm (2.3”) 7-segment displays found on AliExpress to make a clock that can be seen from some distance. When stacked up, this 4-digit display roughly doubles the height of an apple used for scale, and to put things together elegantly, he implemented a couple interesting techniques.
First off, he’s using one of Microchip's new ATtiny series, the ATtiny3216, which has enough I/O pins “to drive the displays in a straightforward way.” This chip features RTC functionality that can be controlled by an external 32.768kHz crystal, allowing for a simplified driver circuit consisting of the microcontroller, crystal, resistors, and a few capacitors. A single button is provided to actually set the time.
The display measures in at 95mm x 140mm (3.74” x 5.51”), meaning it’s too big for a prototyping board. This also means that that an appropriate PCB would be expensive. Instead, he designed a single control/power PCB that sits in the middle of the clock, with two “outrigger” boards that are linked up to it with ribbon cable. These are used to connect up to and power the display units directly, keeping things nice and tidy.
