Elliot Carter published a tutorial on making a photoresistor-based tachometer.
The project came about when he needed to test the top speed of a stepper motor. After putting it together the project became an important diagnostic tool for a larger project – not only does it test the top speed of stepper motors but it also works as a tool to verify code.
The tachometer measures rotational speed using only simple light sensor with an op-amp comparator circuit. While many mechanical engineers might cringe at the thought of an op-amp, Elliot’s team found a tutorial that explained them well.