This one uses LEDs as the light sensor and mosfets to drive the output to rotate on a single axis. He even reduced the duty cycle on the fets so no heatsink is needed. Mounting in a peanut butter ...

Anchor] Korean researchers have developed a wearable sensor that can measure glucose in sweat.The developed sensor changes ...

Or you can go back to basics and build this reflective RGB sensor from an LED and a photocell. The principle behind [TechMartian]’s approach is simplicity itself: shine different colored lights ...