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automated timer for photo printing

WARNING: This project was put together in around five hours in one day in late March using whatever I had lying around, so it can't be referred to as anything like well engineered or optimised, so be warned.

See it in action:


I do a lot of film photography, mostly black & white so I can develop and print myself (Color film and prints require a temperature of 37.8°C±0.15°, and I don't have any way to reliably temperature control water other than mixing hot and cold water and hoping I get it right, so I don't even bother). I have a makeshift darkroom in my basement, but the main thing I was lacking was a good timer, and I'm particularly prone to mess up exposure time when manually turning my enlarger on and off. I saw that they have enlarger timers, but they are prohibitively expensive and they would be literally trivial to build. I've had 2 of these switchable power relays for a while, and had been itching for a project that uses one, and this was the perfect opportunity. I decided to expand the scope so that it could time every aspect of printing, for the convenience and automation that I clamber for in everything that I do often.


I decided that I would make it with an Arduino for ease of slapping it together, and I had an LCD going unused that would be perfect for displaying time, etc. However, I knew I'd have to turn off the blue-white backlight of the LCD when actually printing, but luckily I had a red LED bar graph around. I tested it to make sure it was safe printing by just having all of the LEDs on it turned on while printing a photo, and it worked. With the relay already picked out, I just had to come up with a way to actually be able to customize the timers. I settled for a DIP switch to enter in stuff in binary, as I didn't have any other way of conveniently getting numbers into an Arduino.

bill of materials

This is everything I personally used, but feel free to substitute parts, because everything here was just stuff I had and wasn't using for anything else. I also linked to the first place I found it, not necessarily the best or cheapest place to get it.


With all of the parts gathered together, I started slapping them together into a workable form as quickly and easily as possible. Pulled out the largest unused breadboard I had so I didn't have to worry about space, wrote down which I/O pins would do what, then realized that I didn't have enough.


terrible wiring setup
terrible picture for a self-proclaimed photographer.
the red and black wires coming in from the bottom go to the power relay.

So, I could either order an LED driver or I could just figure out a way to save I/O pins. Can I cut one of the bar graph LEDs? No, it'd be nice to keep all ten of them, even though they're most of what's eating up my pins. Next step: move the LCD Serial from SoftwareSerial to hardware serial pins. There, saved two pins, as long as I'm careful to not mess with the LCD serial while uploading new sketches. Cut one of the DIP switches, so I have seven switches + one select button, saved another, but I need just one more. Is there a way to have a TX only serial? I only use TX for the LCD, but using serial RX initialized to pin 0 prevents me from using it as a normal pin. After some research, I found SendOnlySoftwareSerial, exactly what I was looking for! After moving the LCD TX to pin two so it's not on hardware serial, I can then hook up the relay control and as many of the DIP switches as I can fit.

Okay, with the pin issues sorted, I proceeded to wire up everything. After my brightness test with the bar graph I decided to slightly dim them with 2.2kΩ resistors, because even though they weren't bright enough to mess with the print, they were just a little too bright for my taste when looking at them in the dark.

pin layout

I was going to put this in an actual circuit CAD program but that would have taken longer than this whole project took to build, and the layout is rather simple and can be devised by my pictures and this diagram alone.

The final pin layout is:

pin # connected to
0 One side of power relay
1 DIP switch 1
3 DIP switch 2
4 DIP switch 3
5 DIP switch 4
6 DIP switch 5
7 DIP switch 6
8 DIP switch 7
9 Select button
10 Bar graph + 1
11 Bar graph + 2
12 Bar graph + 3
13 Bar graph + 4
A0 Bar graph + 5
A1 Bar graph + 6
A2 Bar graph + 7
A3 Bar graph + 8
A4 Bar graph + 9
A5 Bar graph + 10

Bar graph - 1–10 → 2.2kΩ resistor 1–10 → GND
Other side of DIP switches 1–7 → GND
Other side of Select button → GND
Other side of power relay → GND
LCD + → 5v


Get the code here

very inefficient code
i'm sure that it's literally impossible to make this code any better.

Like with the hardware, I slapped the code together. I originally adapted code from another project that I haven't written about yet, where there's a global variable for the current program mode, and in the loop it switches through the modes (like a Finite-state machine), but it kind of got out of hand with the number of modes and I never bothered to refactor it into something that makes more sense. It works, and it's not like it needs to be super efficient because a few extra ms delay doesn't matter in anything but the actual timers themselves.

The modes are cycled through using the Select button. I actually turn on the relay when it is first turned on, to allow for manual switching of the enlarger to view negatives, get it sized and focused, etc. I also turn off the LCD for that time as well to make it easier to see as the LCD can be quite bright. Then I switch into the modes to set the values for each of the timers.

If the input is set to 0, then it will preserve the last used value (even through power cycles thanks to the Arduino's EEPROM!) to allow for easy repetition of the same times. All of them are inputted in seconds except for the fixing time, as I fix for 4–5 minutes and since I can't use all the DIP switches I can only go up to 255 seconds, which means that I can't go higher than 4.25 minutes. Fixing doesn't need to any more accurate than the minute for me anyway, so I'll leave it like that. I also decided to exclude adding in a wash timer, as I don't usually time my washes any more than "it's been over xx minutes" and I usually start developing another image while the last one is being washed, and I don't want to have to wait around for a wash timer to end.

After all the timers are set (or preserved) I then turn off the LCD backlight and prepare for exposing the frame. Pressing Select again begins exposure, and then continue to press Select to move through all the timers, where it then loops back to the beginning.

still to do

I don't really plan on working on this project more, as it works perfectly fine for my purposes, but if I do decide to continue, this is what I'll probably be doing (in order):

  1. Replace the small pushbutton for select with a Jeopardy-button–like thing so I can click to the next mode without having to have someone else press it or drag around the whole contraption with me.
  2. Add an LED Controller to free up I/O pins to make it easier to code and build and so I can use all of the switches on the DIP.
  3. Turn it into an Arduino shield or PCB so it doesn't look all ugly on a breadboard and is much more compact.
  4. Add a 10-digit keypad (probably could rip it out of an old phone or something) to make entering specific values easier.