I need a tachometer to calculate some mechanisms. For example, to calculate the speed of a robot vacuum cleaner, you need to know the rotation speed of the electric motor and, based on this, calculate the gear ratio of the gearbox.
I present to your attention a homemade 3d printer. Assembled from plywood, aluminum, plastic and Arduino. This printer is unique in that its kinematics is peeped from Ultimaker, but done with quality modifications.