Compensating Development Timer
Developing traditional film and silver paper is very temperature dependent. The warmer the developer solution, the less time the film or paper needed to be developed and vice-versa.. Historically, people read the temperature and then manually corrected their development time accordingly, using a correction table provided by the manufacturer.
It turns out that these corrections for temperature are quite similar across different manufacturers, although the corrections are different for film and paper.
Many years ago, a company called "Zone VI" realized this and created an analog timer that corrected for this effect. You placed a temperature probe into the developer and it corrected - via analog adjustments - what a "virtual second" actually had to be. The photographer just looked up the normal development time for 68F developer and the timer ran faster or slower based on the actual temperature. Better still, if the temperature of developer varied during development, it corrected for that in realtime. The Zone VI Compensating Timer had settings for film, paper, and realtime.
The timer was a work of genius engineering and a really nice addition to the serious photographer's wet dakroom. I've depended on one of these for years to make my darkroom work repeatable with minimal thinking or measuring. Mine is getting kind of old now and I began to wonder what I would do if it broke. The timer did come with a "Lifetime Warranty", Sadly Zone VI and its founder, Fred Picker, are both now long gone making warranty claims ... difficult.
While I could design an analog replacement or just figure out the circuit of the Zone VI, it occurred to me that it would be easier to just design a "work alike". Thanks to the explosion of interest in robotics and the Internet Of Things, there is an embarassment of riches of computers, sensors, switches, temperature probes, and so forth. Not only can we build something like this ourselves, doing so has several advantages over the old Zone VI timer:
* It's digital, not analog, so we don't have mess with
a bunch of precision parts and corrective feedback circuits.
* It's software controlled so you can customize how this
timer works to suit you. Don't like my compensation factors?
Want to adapt this for a different application? Both are
easily done with software changes.
* It's cheap. You can build one of these for well under $50.
(The original Zone VI timer was around $200 if memory serves,
and that was when money was still worth something. :)