[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.

The code includes two Arduino libraries: one for the servo and another for the ultrasonic sensors ... a lightbulb controller using a light-dependent resistor (LDR). The LDR continuously monitors ...

[mircemk] shows how to create a simple non-contact proximity sensor using little more than an Arduino Nano board, and a convenient software library intended to measure the value of capacitors.