We all should have a couple of laser pointers sitting in the "used to be cool toy box" There are many uses for those lasers other than the usual pointer application.
Detecting things:
I have used them for generating an electrical signal when detecting things. I needed a flow meter to detect the flow rate of the heat exchanger for my solar heating system. To buy a unit with high accuracy, and a readout, was in the several hundred dollar range. I found this cute German made home water meter for less than $100. It only had one pulse output for every gallon, which was not sufficiently accurate to measure the sub gallon per minute rates that were sometmes necessary. I mounted a laser pointer so that it shined through the notches on the rotating disk that was right on the flow turbine, so it turned hundreds of times to each gallon of flow, and gave multiple puldes per revolution. I mounted a photodetector in a blackened tube, and glued a lens on the front of the tube. The detector assembly was focused on the notches of the rotating disk. and was able to generate a pulse stream for each rotation. This pulse was buffered, amplified and converted to a digital pulse that was read by the controlling computer to regulate the speed of the pump to produce a constant flow rate.


Detecting a rotating disk