Texas Steampunks & Neo-Victorians
I've been experimenting a bit with using infrared (IR) phototransistors and LEDs as a proximity detector circuit for a possible future project.
The test circuit is a simple one but adequate to test the parameters I was curious about: waving a hand over the IR parts while monitoring the voltage across the transistor with an oscilloscope proved that it was working.
I noted that there was quite a bit of 60 Hz ripple present that vanished when I waved my hand over it and realized that the ripple I was seeing was the result of the ambient fluorescent lighing. That meant I needed an IR pass filter before I could continue the testing.
I searched the internet and tripped across what looked like a low cost solution: unexposed but developed 35mm photographic transparency film. Then I looked up the price of transparencyfilm and found it wasn't such a low cost option after all.
Reasoning sideways, I decided to try exposed (and developed) 35mm print film since it's basically the reverse of slide film. I also had a bit on hand: the couple of leader frames found on every roll of 35mm negatives.
Exposed 35mm print film is a very dark brown with about the same degree of transmission as an oxy-acetalene welding filter. The difference is that the 35mm film is designed to pass infrared light so it won't warp or melt in the enlarger. I was hopeful it would function work at a adequately for testing.
Testing showed that the print film works just fine as an IR pass filter!
There was a small measure of ripple across the transistor (partly from a lack of shilelding) but I was getting a pretty clean transition from cutoff to saturation across the transistor (about 5V with a 6V supply) and that's enough to work with.
So, this was a twofer: the IR proximity sensor works and the DIY pass filter is actually quite effective!
<Disclaimer: No animals were mutated beyond recognition during these experiments.>