Add an OSHW certified stopwatch to your toolbox
[MakingDevices] has created a simple stopwatch that is a great introduction to surface mount electronics design and assembly. The project is certified open source hardware (OSHW), with Gerbers, KiCAD files and software all available.
Conceptually, the stopwatch is simple, with a row of two four-digit seven-segment displays driven by a PIC18LF14k50 microcontroller via multiple NPN transistors. The PIC doesn't quite have enough data lines to drive both displays at once, so an inverter is used to switch between the two seven-segment blocks.
The circuit is continuously powered by a CR2032 button cell battery. For normal use with display, [MakingDevices] estimates over 30 hours of operation and over 140 hours without display, but always counting the time. When idle, the PIC's "Extreme Low-Power (XLP)" capabilities put operating window estimates well beyond the self-discharge of the coin cell battery. There is an in-circuit serial programming (ICSP) footprint that accepts a TC2030-MCP-NL pogo pin adapter to flash the PIC.
Don't let the simplicity fool you, this is a well-researched project with in-depth articles on design, simulation, and battery consumption. Various videos and glamorous shots give a complete picture of the process, from design, assembly, testing to final validation.
It would be wonderful to see the project expanded or hacked further, perhaps with a nice box or case.
[MakingDevices] has created a simple stopwatch that is a great introduction to surface mount electronics design and assembly. The project is certified open source hardware (OSHW), with Gerbers, KiCAD files and software all available.
Conceptually, the stopwatch is simple, with a row of two four-digit seven-segment displays driven by a PIC18LF14k50 microcontroller via multiple NPN transistors. The PIC doesn't quite have enough data lines to drive both displays at once, so an inverter is used to switch between the two seven-segment blocks.
The circuit is continuously powered by a CR2032 button cell battery. For normal use with display, [MakingDevices] estimates over 30 hours of operation and over 140 hours without display, but always counting the time. When idle, the PIC's "Extreme Low-Power (XLP)" capabilities put operating window estimates well beyond the self-discharge of the coin cell battery. There is an in-circuit serial programming (ICSP) footprint that accepts a TC2030-MCP-NL pogo pin adapter to flash the PIC.
Don't let the simplicity fool you, this is a well-researched project with in-depth articles on design, simulation, and battery consumption. Various videos and glamorous shots give a complete picture of the process, from design, assembly, testing to final validation.
It would be wonderful to see the project expanded or hacked further, perhaps with a nice box or case.
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