I tried to design a timer that would do everything it needed to do but with the smallest number of pieces and simplest mode of operation. It only needs the PIC, a four digit LED display, one other IC, a resistor network, one pushbutton switch and a capacitor. It can run on batteries if you use a solid state relay to turn the exposure light on and off thereby adding a minimum of parts to make a fully functional darkroom timer. A simple regulator would add only a few extra parts and allow the use of a `wall-wart` for power.
All the functions are in firmware, allowing the use of one switch for control. I sketched in the schematic in Eagle format and here's a jpg image, too. This text file has the parts list and some notes to help you build the timer. Here's the firmware. You