Darkroom Ratio Calculator

From Hackstrich

Calculating ratios in my head when mixing/diluting chemicals in the darkroom is hard, so I want to build a box to do it for me.

Project Status

  • 2015-09-13: Was a fun design project but can't justify spending the money to get it built right now. Calling it cancelled for now, maybe someday I'll pick it up again :)
  • 2014-12-14: Completed routing board, needs silk and checklisting still.
  • 2014-12-13: Continued routing board.
  • 2014-12-12: Completed and checklisted schematic, started routing board
  • 2014-12-11: Started building BOM and schematic, put together first ideas for a front panel design.
  • 2014-12-08: Started putting together initial idea/design.


  • Input via rotary encoders
  • Output on red 7 segment LED displays
  • Unit will have a light sensor and dim the displays when the lights are turned off so they're safe
  • "Fields" are:
    • Ratio Component A (3 digits) w/ encoder
    • Ratio Component B (3 digits) w/ encoder
    • Value Component A (4 digits)
    • Value Component B (4 digits)
    • Total Value (4 digits) w/ encoder
    • Round To (2 digits) w/ encoder
  • Ratio Components, Total Value, and Round To are settable, Value Components are the output
  • Round To is a way of automatically adjusting the total to find the nearest value that makes ratio components rounded while staying accurate
    • Measuring 67.2ml of liquid is a pain
  • Total required I/O is:
    • 20x 7-segment displays
    • 4x rotary encoder inputs
    • 1x reset button input (resets to 1:1, total 1000, round to nearest 5, or something like that)
    • 1x light sensor input