[Gord Payne] has one with an elegant sensor ... reaching the LDR from the LED decreases as the pipe is bent, and with a simple divider circuit a voltage can be read by an Arduino.

Here, a standard light-dependent resistor (LDR) is used as an analog input sensor and directs its output to the Serial ... After uploading the Arduino code (as usual), just copy/paste the Processing ...

This solar tracker system uses the Arduino UNO board ... In the setup() function we set the pins were the LDR are connected as INPUTs and position the servo motor at 90° then wait for a 2 seconds ...