Arduino diy open-source darkroom f-stop timer

This article explains briefly why using f-stops as timing units, instead of seconds, does make a lot of sense in the darkroom, and why I decided to build my own timer instead of purchasing one.

The timer in action video here

Why f-stops are better than seconds, imho?

Well, f-stops are not better than seconds, any timing device and charts do the job well. F-stops are simply way more practical in the darkroom. In fact, f-stop for the darkroom is the perfect companion to the Zone System as it allows to keep using the same rationale and reference system, from shooting to printing, from the light-meter to the enlarger, or from the very first sight at the scene to the final dry print. It also greatly simplifies the darkroom maths and test prints, especially if using the split-grade method.

Much more by Gene Nocon

Said that, since my plunge into the darkroom adventure, I used a collection of dedicated darkroom timers, some basic, some excellent, some over-complicated, some not-too-bad, none of them perfect or at least adapted to split-grade.

  • a loud ticking Swatch over my left dumbo-style ear. Perfect for timing, but no ear flapping allowed
  • some BIG dial analog clocks and timers such as Gralabs and Haucks, very robust, yet clunky and less than precise and/or repeatable, not the best visibility in the dark, no programming feature
  • electronic timers such as Kearsarge, LPL, Durst, Gossen, Wallner, very precise and excellent visibility. The LPL ET-500 and Kearsarge 301 are great tools, very robust and reliable, yet too basic. The Gossen 2000 and Wallner MC505 have awfully complicated interfaces, the excellent Durst Digitim S 1000 being the best compromise, but misses real sequence programming features and uses +/- buttons for setting times which I believe to be simply a great ergonomic swindle.
The excellent by simplicity, Durst Digitim S 1000

Considering the RH Designs StopClock the closest to my new darkroom whim, and having some (long) spare time to recover from traffic accident injuries, I decided to build my own, starting with a pair of to-do lists, inspired by Gene Nocon, William Brodie Tyrell, Henke Petersson, the Timerino guys in Italy and the great Arduino community.

for the timer:

  • dual bright display: f-stop + seconds
  • automate the test-strip process
  • simplest interface/commands,  without +/- buttons
  • sequence exposure program
  • cover, uncover, single exposure modes
  • calculate print enlargement correction factors
  • simplest time input method possible
  • two selectable enlarger outputs ( I use two enlargers )
  • safe (not keen to try 240V Electroconvulsive Therapy)

for myself:

  • learn about Arduino hardware
  • learn the Arduino programming language
  • learn safety best practices for main power supply
  • keep hardware costs as low as possible, possibly under 100 CHF/USD/EUR

Here is the result:

Arduino-based f-stop timer, f-stop upper display, time lower display

What it does as a timer:

  • Simple ticking countdown timer, start/stop & resume
  • Precision < 1% (measured on output relay)
  • 5 channels automated test strips, 11 zones each (remember the Zone System ?)
  • Specific test strip zone recall for countdown
  • 5 channels for exposure programs, max of 11 steps each
  • Automated print scaling time correction
  • Cover, uncover, single mode exposure for both test strips and programs

Some hardware features:

  • Red panel for better visibility under safelight
  • Rotary selector (no +/- buttons)
  • Switched and fused 240V main input
  • Two selectable enlarger outputs (500W)
  • One switched safelight output
  • USB plug for future firmware updates
  • Focus and Start/Stop led switches
  • One jack plug with two serial footswitches for remote start/stop (one per enlarger)


Monkito user’s manual_rev_e

text and images © Elia Cottier 2017 – 2019

4 thoughts on “Monkito f-stop timer

    1. Thanks,
      I am still in the head building process. The LED module should take the bulb place without having to modify the original head, except maybe for a hole to run a cable.

      When finished I wil post a vid and schematics.



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