DIY Light-Pipe as a Touch Sensor
I've been trying to think of a good sensor to mount onto my polypod's legs (Yes, I'm still working on it) to give them a sense of touch. Some people have very creatively using miniature push buttons that seem to work. I was hoping for a more omni-directional solution. The switches probably rely on the foot coming pretty much straight down.
One day I got the idea of using a light-pipe made of thin-walled tygon tubing. The tubing is filled with some clear fluid (in my case 99% isopropyl alcohol) and plugged on one end with an LED and on the other with a phototransistor. I initially used water but it insisted on creating small bubbles in the tube from the dissolved gases. Like an optical fibre, the light pipe transmits much of the produced light to the phototransistor even when the tubing is bent through a 180 degree bend.
When the tubing is pressed against a surface, the tubular shape distorts and this attenuates the light reaching the phototransistor. This causes the detected signal to drop. By pressing the tubing against an electronic scale I was able to produce a graph showing the signal versus the applied force in grams. A 100g force should produce about a 42mV drop in signal (in my setup) and this should be easily detectable by a 10bit A/D.
Once polypod is walking I'll retrofit some sensors onto the legs and see how well it works.
More details and photos can be found at http://www.dr-iguana.com/prj_LightPipe/
Thanks for your interest.