Let's Make Robots!

DIY Light-Pipe as a Touch Sensor

Give a sense of touch to a robot using light

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.

Dr Iguana

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Wow, very creative. Would you get better readings when you are using non transparent tube (just paint it black) to reduce the amount of foreign light?

Thats a really interesting bit of work + research. Love that you have a linear response over an 'omni-directional' range of contact angles. Cant think of another sensor that would do that right now. Very cool.

I thought this was similar to something I saw on HackaDay.


After actually reading the description, I realized I was WAY off.

This is a very interesting solution as has been said by others here. I am definitely collecting this one. I would love to see you send this as a tip to hackaday.

Thanks for sharing.

Thanks for the interest.

Regarding painting the tubing, this would be brilliant if it can be done.   My first guess would be that paint will adhere poorly to Tygon but perhaps there are some types that would work. Perhaps a spray paint?  I think I still have a can at home.  Hmm.

There is another Tygon tubing that is black (UV resistant) but its ShoreA hardness is 64 rather than 55 so would be worse in sensistivity.  I'll try adding a length of shrink tubing over the end with the phototransistor to see if that helps.

There is a softer variety of Tygon called R-1000.  There is a 1/8"ID version but it has thicker walls; 1/16" vs 1/32".  So even though it's ShoreA hardness is 40 vs 55, the thicker walls might prevent better sensitivity.

Dr Iguana


Yeah, i guess that spray paint will not work either since in this tube is a lot of softerner (dunno the English word for this) but i was thin king the whole night to just wrap it in black palstic foil. This should be ok and not decreasing the sensitivity. I am SURE that will increase your output signal a lot.