I was in need to measure the speed of my adjustable power drill (rotations per minute)
so to accurately adjust the speed at the specifications of my new high quality drill bits HSS-G.
I did some searching and my final selection was the DT-2234C+ which haves good resolution and gets up to 99.999 RPM.
I got it back at 2010 for 10 EUR shipped,
it did pass all my tests , inner quality + accuracy .
The base principal about using it, is to install on the rotor one small piece of the included reflective adhesive tape.
And to just point the digital tachometer to it.
Personally I did move one step further and created one small patent, that is this reusable Velcro collard with the needed reflective adhesive tape on it.
This simple idea saves you from wasting reflective tape in to some extent.
The test bed is my high quality turntable, which haves speed control by PLL and also is direct drive
(no belts in it).
The results are more than perfect .
The specific digital tachometer it can perform a measurement from the distance of 20mm up to 50 Centimeter away.
In my tests it managed to measure even from 2 meters away !!