RPM of a small DC motor using a Hall Effect sensor

Hello,

Would it be possible to measure the rpm of a dc brushed motor with a hall effect sensor? I don't want to attach a magnet with the motor. I was thinking of detecting the magnetic field of the coils on the rotor, and calculate the rpm from that.

Thanks.

Adding magnets symetricaly is used on mills and lathes spinning up to 2500 RPM.
Just get Yourself a sensor and test if it detects the windings. Else reflective optical sensors might be a choice.

I understand that, however, the dc motor that I am trying to measure the rpm is a very small motor and adding magnets to the shaft would cause a decrease in the rpm.

2 or 4 magnets shouldn't slow down the RPM noticeably only the acceleration.
What about a reflecting surface, one black, one white in a practical place, and an optical sensor?

How about simply holding a HE sensor next to the motor and see if it lights up?

I have used a QRE1113 reflective optical sensor to measure the speed of a small N20 DC motor. I cut a small slice off a plastic knitting needle (8mm diam IIRC) and drilled a hole in it for the motor shaft. I painted the plastic disc matt black and put a spot of white paint on it.

The code in this link is derived from the program I used with that motor. It worked fine up to 16,000 RPM and I have no reason to think it would not work at higher speeds.

...R