f/stop timer

Hi all,
thought I'd join up here to post my arduino project which is an enlarger timer that operates in EV instead of linear seconds ("f/stop timer"). Without going into the gory technical details of wet printing, this approach is considered The Way but is hard to do with a simple linear timer. You can buy an f/stop timer but they're a few hundred dollars.

It's not a particularly complex thing (the electronics are trivial), it's just lots of
menus and number juggling. I did it with an arduino because that would make it easier for others (analog photographers; they're not much into the microelectronics) to duplicate without requiring much soldering, custom PCBs or ICSP cables.

Source is here along with a basic user manual (most people building this won't read or understand the code; they'll just want a timer). It includes an SMS-style alphanumeric entry class; license is GPLv3.

Photos coming later (I'm about to go on a massive holiday) but in the meantime, let me know what you think...

Added video demonstrating the user interface while it's still on the breadboard.

May sound like a strange comment, but your LCD looks really cool! I always use boring black & white(ish) LCDs, with no backlight or anything else fancy. The red-on-black screen looks very professional.
Any explanation of what an f/stop timer is? A google search did not provide anything of particular interest or usefulness :(. (Darkroom Automation Fstop Timer).


Sorry, f/stop printing is a means of choosing enlarger times based on a logarithmic scale; it's an advanced black & white darkroom technique.

The LCD is red so that (once covered with rubylith) it can be seen in the darkroom and not fog the paper, which is insensitive to deep red light. $7 on eBay :wink:

I see, so f/stop timers change the amount of light you expose photographic liquids to, in the same way the exposure time on a camera changes how bright the picture is? You sound to have had a good deal on the LCD; I will have to have a look for one. :slight_smile:


Yes, photographic paper is negative, just like B&W film; when you expose the paper through the negative in an enlarger, you end up with a positive image. The longer you expose, the darker it gets so you generally want to dodge (lighten) and burn (darken) some areas, hence the timer.

The term "f/stop timer", while being the agreed jargon, is a really bad/misleading name. It should be called a logarithmic timer or similar.

Nice project. I miss my darkroom! Sigh!

Good to see someone else has found a good use for the ribbon cable from old IDE leads :slight_smile:

Update: I've made a custom PCB for this project that implements the user-interface functionality:

  • 4x4 keypad
  • LCD with PWM backlight dimming
  • rotary encoder with pushbutton

It has a couple of output bits as well as a single relay driver (BC548 with protection diode for inductive loads like relay coils). The board is not specific to the timer, it's just a rich user-interface shield with limited output. Mostly it's just plumbing to all the input devices and there's not much on there in the way of actual functionality.

If you go check out the construction link you can see the board and download the (GPL) design files. I have a couple boards still spare so if you want to buy one, email me.

Red on black displays here too with serial interface options.