Rpm sensor

Hi i am looking to see if it is possible to find a way to use and rpm sensor that is small and portable that will output a dc voltage or current directly proportional to the actual rpm it measures so that i dont need a tacho for a measurement device i am thin king of using a hall sensor but dont know if there are any products to suit? any help or advice etc would be greatly appreciated

For a Hall sensor to work a magnet would need to be attached to the spinning device. If you want something that’ll make an analog voltage to measure, a car tachometer taken apart just might work when you add a voltage divider if needed. Another sensor would be a phototransistor and a laser pointer aimed at a spot on the shaft with shiny and dark sections. At the shiny spot, the laser beam hits the shiny shaft and the reflected light turns on the phototransistor. A shaft encoder works the same way but with an LED instead of a laser.