Sensors?

My head has been wrecked thinking of how I can get a webcam to pick up the right motion in the horizontal wall. I decided to look elsewhere to see what other things I could use to detect this. I was using one of those paper towel dispensers with the sensor in the front and thought that I should look into that as it seems the perfect kind of technology for what I need.

I had a look around the internet and found the following website with something that might suit, http://www.glolab.com/dp-001/dp-001.html -- although that says one of them costs $22.95 and as I need 40 (one for each circle) it would be too much cost wise. I am currently having another look around to see if I can obtain these sensors cheaper or maybe make something similar up with other materials.

If I do use sensors for my project it will also mean that my project would be portable which will mean I will have a greater option of user testing.

My father works for a training company who are currently doing a course on security and monitoring. I was asked to digitize some technical drawings for them and noticed that they also use some types of sensors for their work. I plan to ask the person organising that course a few questions about sensors and see if he has any suggestion on how I could use sensors in my project.