Changes from prototype to production - lawrencebillson/parkrunStopwatch GitHub Wiki

Changes / requirements

The following changes will be essential:

  • Design for reduced power consumption
  • Monitor the battery voltage input into the DC/DC converter with an ADC - show battery voltage/percent full

The following changes would be nice:

  • Use a larger EEPROM - 4096 bytes is sufficient for 1365 results (3 bytes per runner), a 65kByte unit costs a few cents more and stores ~20,000 times

Board Layout Notes / Thoughts

  • The boost converter is going to be SMD, I may as well make the whole board SMD
  • I'd like to use an 'off the shelf' enclosure to make this easy to reproduce around the world
  • Target is a two layer board
  • I'll use standard AVR programming header pins - this will minimise difficult with flashing the ATmega
  • On-board buttons are OK for everything except for the 'over the line' button
  • Use digital IOs rather than the LCD shield analogue value buttons - I can then use interrupts to process the results

Choices

RTC / EEPROM - Make versus buy

Make:

  • Easy to get a larger eeprom (more runners)
  • More reliable supply chain
  • Parts cost of more than $5
  • Smaller - all SMD

Buy: (Current inclination)

  • Fixed EEPROM size (~1200 runners)
  • Low cost $1
  • Supply chain seems OK
  • Needs to use through-hole with an odd pitch

LCD - source / type

These are comparatively expensive (~$10) from Digikey. A suitable display can be had from eBay for closer to $3 at the expense of:

  • Less reliable supply chain
  • Less choice - they all seem to have backlights rather than being reflective/transflective

Current thought is to use the eBay supply.

Enclosure

This whole thing is predicated on having a good enclosure with nice buttons. This will need a lot of thought!

Serial interface - Make versus buy.

Again, these can be had very inexpensively from eBay. Further thought is required.