Yes, I actually have built the brodie-tyrell f-stop timer. Its fantastic as an enlarger timer and a great build. It even has facility for density measurement (which I haven't implemented).
Also, there are a good few apps out there for smart phones for process timing - I use one called darkroom timer (android) that's free and very user friendly. Having said all that, phones are bright - and even more so if they ring when processing, say 5x4 film in open trays etc. I still think there is good space for doing an arduino process timer. If I had my choice, I would have a temp compensated one as mentioned above, possibly with a an added metronome function as I've taken very much to using that for dodging and burning.
So, I'm rusty on darkroom experience, and was never one for playing with different temperatures in processing -- I've always tried to adhere to 68 degrees. Is it generally safe to assume that temperature compensation is linear? That is, if you graph the time vs. temperature on most developers, that you'd come up with a pretty much straight line? That is the case for XTOL (which is what I've used lately), but I haven't gone out to survey all the developers out there. I do really like the idea of having temperature compensation build in to the timer, though I haven't pursued the extra code yet, nor investigated the best way to sense temperature.
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