Design Notes
Revised Requirements — Binary Wristwatch
- Wristwatch form factor with simple 3D-printable case and strap hardware handled mechanically.
- Replaceable coin-cell battery power, assumed CR2032 unless changed.
- Simple binary LED display: LEDs should be physically large/bright enough to read as lit bits in a stylized 0/1 layout.
- LEDs should normally be off; user presses a button to wake/display time for a few seconds to preserve battery.
- Mostly simple design with some visual flair: recommended layout is two rows or arcs of LED bits, with silkscreen labels or printed case windows.
Architecture
- CR2032 holder -> reverse-protection / battery rail -> low-power MCU.
- MCU uses 32.768 kHz crystal for accurate timekeeping, or an external RTC if MCU library availability forces it.
- LED display driven directly by MCU GPIO through current-limiting resistors at low duty cycle/current.
- Buttons: DISPLAY/WAKE plus optional SET and MODE buttons for setting time.
- Programming/debug connector included as pads/header.
Power Budget
Assumptions:
- CR2032 nominal 3.0 V, typical practical continuous current should stay low; LED display is momentary.
- Low-power MCU sleep/RTC: target about 12 mA plus MCU active current.
- Display timeout: 3–5 seconds per button press.
Sizing target:
- No boost regulator for simplicity; run directly from coin cell where possible.
- Avoid high LED current to prevent CR2032 voltage sag.
- Use high-efficiency red/green LEDs rather than blue/white, because blue/white forward voltage may be too high from a depleted coin cell.
Display Encoding Proposal
- Hours: 4 LEDs for 1, 2, 4, 8 values (12-hour binary-coded value).
- Minutes: 6 LEDs for 1, 2, 4, 8, 16, 32 values.
- Optional accent/PM LED if GPIO budget allows.
Design Decisions
- Prefer a very-low-power MCU such as ATtiny/AVR, PIC, SAMD low-power, or similar available library part.
- Prefer external 32.768 kHz crystal if the MCU supports it for timekeeping.
- Use SMD parts by default to keep the watch thin, but choose larger 0805/1206 LEDs for readability and beginner-friendly assembly.