[MakingDevices] has created a very simple stopwatch that tends to make for a great introduction to area mount digital layout and assembly. The challenge is open supply hardware (OSHW) licensed, with Gerbers, KiCAD documents, and computer software all readily available.
Conceptually the stopwatch is straight ahead, with a row of two 4 digit 7-phase displays becoming pushed by a PIC18LF14k50 microcontroller by several NPN transistors. The PIC does not very have enough information traces to drive the two displays at the moment so an inverter is applied to toggle among the two 7-section blocks.
The circuit is continually powered from a CR2032 coin cell battery. For usual use with display screen, [MakingDevices] estimates 30+ hrs of operation and 140+ hrs without the need of show, but even now counting time. When idle, the “Extreme Small-Power (XLP)” abilities of the PIC set the functioning window estimates perfectly beyond the self discharge of the coin mobile battery. There is an in circuit serial programming (ICSP) footprint that accepts a pogo pin TC2030-MCP-NL adapter for flashing the PIC.
Never permit the simplicity idiot you, this is a properly documented project with detailed posts about the layout, simulation and battery usage. Numerous video clips and glamour pictures give a full photograph of the process, from layout, assembly, tests to remaining validation.
It’d be amazing to see the job extended or hacked on additional, perhaps with a cute enclosure or situation.