Been thinking to make a quadcopter, not a huge one, a fairly small and light one to start out with. The problem is I would like it to read the RPM of each engine separately. There are several ways I have came up with: - One of them is to use the current reading and calibrate it, however using this would result in an estimate and not a continuous update. - The second is to use a photo transistor sensor, which honestly would be way easier, but with uncertainty of the movement of the blades it may damage the blades. -The third is using a hall effect. With the hall effect I know that there are two magnetic sources from which the hall effect might detect. One of them comes straight from the static magnets on the brushed DC motors, and the other the coil, as it is designed to just spin around and wired in such a way that it always has an opposite polarity than the magnets. I would need to filter out the source of the magnets using a capacitor, as for all intents and purposes it is a constant voltage source, and the coils move in such a way I would get an oscillation exactly in the frequency of the rotation. The question is, would it be a valid solution to use a hall effect to detect the rotation, and where would be best to place the sensor in relation to the motors? Again the motors are brushed, with static magnets and moving coils.