motor tachometer

[dethrow55]

hello has anyone hear built a tachometer for ac motor that works with the touch dro? looking for some idea thanks

[Yuriy]

The easiest would be to mount disk with some magnets on it (or the spindle) and add a Hall effect sensor or something similar.
I'm assuming you are not talking fixed RPM motor (or you would not need a tachometer). If it's a 3-phase DC, you can do some creative stuff with a clamp-on current probe, but that gets into "I really know my op-amp circuits" territory.
What is your setup, and what are you trying to achieve?