how does a non-contact digital tachometer work?

Answer 1

A piece of reflective tape is placed on the shaft(but may not be necessary) The Tach has a light source and a light sensor on the business end of the tach.

Pointing the light/sensor at the rotating shaft the computer inside detects the reflected light. The computer times the period between the reflected light pulses (time per single revolution) and then computes the revolutions in a minute and displays the result on an LCD

Another type of noncontact sensor uses a Hall effect transistor to detect changes in magnetic fields. This type is not as common for a test TACH but would be found on a dedicated tach system. Like a breakerless ignition sensor in a distributor

Answer 2

a contact tacho has a sensor such as a shaft which rotates with the moving object and from this rotate a number or amplitude is generate depending on the number of rotations. a cam or bump on the shaft will indicate when it has rotated full circle (360 degrees). counting these will indicate the revolutions per minute.

a non contact tacho works in a similar way, rather that being physically connected a magnetic sensor or light sensor indicates a point on the shaft, so that when it rotates 360 degrees full circle then the count is raised from this revolutions per minute can be calculated.

Answer 3

You could put a piece of reflective tape on the rotating part and use a light source and a light sensor to detect the reflected light. You would then have to connect the light sensor to a pulse counter and some other electronics to scale the signal from pulses per second to RPM. (Or you could just pay the guy £72 per hour)

Answer 4

typically reflected light or magnetic pulse or encoder or mechanical...a number of ways depending on application requirement and readout needs. ie, digital or analog