The guy in the video used 3 photo resistors and calculated difference between the center and each one on the side.
I tried this method but found that the all photocells are not created equal in that their resistances are not uniform under the same light levels thus if one photocell had more light hitting it, it could still have a higher resistance than one with less light hitting it. I thought of mapping the ranges of each photocell to values of say 1 to 10 but then came across a method which makes it 'obvious' which direction is lighter or darker. It's done by placing a tall light barrier between two photocells (e.g. vertical barrier between the two photocells used for east-west). If the two photocells are not directly facing the light source, one of the two photocells will be in the shade of the barrier, making the difference in resistance between the two 'obvious'. The inherent differences in resistance between the two photocells under the same light source can be ignored by throwing away any values that are relatively close and only paying attention to values that greatly differ.
- Scotty