A few years ago, I decided that I wanted to get back into photography. Lots of things had changed between 1972 when I stopped doing photography. I still had my Leica M4 and some great lenses but my favorite film, Kodak Panatomic-X had been discontinued. There was a new film though, Kodak Technical Pan, that looked promising. I had kept my old Nikkor stainless steel reels and tanks that, after all these years, looked brand new. Kodak offered developing chemistry for Tech Pan so I went ahead and bought some film and chemistry and tried it out. Results were OK using my Nikon LS-2000 scanner.
As I was doing my test rolls, I thought it would be nice to have three things. 1-a good exposure meter, 2-an accurate thermometer and 3-an accurate development timer. After some thought, I believed that I could make one small device that would perform all three functions. Building it would involve designing a circuit, printed circuit board, writing software, and finally building a case for the thing. It would be have to be small and light so that it could be carried on my person at all times. It should also use a battery that is commonly available everywhere.
Choosing a microprocessor was easy. I had done a lot of work with the 8051 and still had a working copy of the Intel PL/M-51 compiler. Texas Instruments had a nice light sensor chip, the TSL-220 that would suit my needs. I also found a serial temperature sensor chip and mounted it in an aluminum tube which would double as a stirring rod. An off the shelf 16 character LCD display would provide the human interface
The first step was to design the schematic. I used a simple schematic capture program that allows you to drop parts onto a page and connect the pins by drawing lines. Once this is done, a companion program produces a PC board file ready to email to the prototype house. The software has to be considered at this point in terms of which pins will be connected to which peripheral chips.
When the board arrived from the prototype house, the parts were soldered in along with a socket for the microprocessor. Programming the microprocessor is done with an adaptor that attaches to the desktop PC. The first code you write is typically to send something to the display like the ever popular “hello world”. Once you get the display to work, it is on to writing the rest of the functions. The light sensor chip produces a series of pulses. The faster the pulses, the more light. Connecting this pulse output to one of the interrupt inputs on the microprocessor provides a good way of measuring the light. Every interrupt causes a number to be incremented. Another interrupt can be set to occur at a regular interval, such as every second. On that interrupt, code can be written to check the number accumulated by the light sensor interrupt and look that up on a table that corresponds to measurements taken by a calibrated meter to produce a meaningful measurement in terms of shutter speed and f-stop for a given film speed.
When the temp sensor is plugged in, the microprocessor senses its presense and goes into time and temp mode. With the film in the tank and the developer at the right temperature, pour the developer into the tank and press reset on the meter. The display shows minutes and seconds counting up as well as the current probe temperature.
It took a few months, but I finally got everything working. As things go sometimes, another project came along that took my attention. By the time I got back to it, Kodak had discontinued Tech Pan but did come out with a black and white film that could be developed using C-41. That meant I could take it to my local Wal-Mart and have it processed for around a buck. It no longer made any sense to develop film in the kitchen. Also, there were some beautiful light meters (some in mint condition) on Ebay for a fraction of their original cost now that everyone was flocking to digital cameras. I picked up a really nice one and I’m happily using it to take great photos with my old Leica.
