Ever wondered how it is that elevator doors these days have the kindly
habit of not crushing people... well its all about laser tripwires.
We build a laser tripwire by using an LDR which is connected to an Arduino analog pin through a voltage divider. Thus the reading on the analog pin varies with the light. Next we point a laser at the LDR and write a bit of code that says if the value goes down then trip the Alarm!
•Light Dependent Resistor