im starting to add lazer diodes, they will let you detect when something is blocking a lazer. Kinda like a sight sensor, but no sight.

Ive been trying to figure out how to make the lazer, and ive got a good start as you can see.

In the second image you can see the lazer, (left,) and the receiver you get from wrenching it, (right.) With the advanced versions you will only need one lazer.