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Rockler Digital Master

Essentially, the sensor attached to a sleeve fits over the router's height adjustment shaft. As you lower or raise the router the shaft rotates and turns the sensor. The system keeps track of how many turns and knowing the TPI of the screw converts number of turns to height.

By zeroing when the bit top is level to the table, you can set the bit height (depth of cut) precisely.

 

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Then
I guess the "spare" probe may be used for measuring the router's RPM?

 

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Maybe?? I couldn't seem to make it do anything

Usually you need to put a reflector strip (e.g aluminium foil or white paint) somewhere on the spindle to create a pulse when the probe shines shines at it. That will measure the RPM.