Lasted edited by Andrew Munsey, updated on June 15, 2016 at 1:11 am.
Tools for measuring rpm (revolutions per minute) of a rotating device
You can also use a digital bicycle speedometer and do the math for the
diameter of your wheel.
When you have magnets involved in your rotating device, a magnet-based tachometer will most likely not work because the magnets in your system will drown out the magnet of the tachometer (e.g. in a bicycle speedometer).
There are Optical Tachometer kits for $28, you might be able to get them
cheaper if you look around, or there may be a circuit diagram that it posted
to build it from scratch.
Here is the link to a kit..
Also I bought an optical tachometer on eBay for $50, so that is a source as
I recently purchased a Laser Tach for $49 from Marlin P Jones. I like it very much.WWW.mpja.com , Tel 541-848-8236
| 15160 TE|| LASER DIGITAL TACHOMETER
Hand held photo reflective tachometer uses LASER beam. 5 digit LCD display measures RPM from 2.5-99,999. Resolution: .1RPM /2.5-999, 1RPM/above 1000. ... Requires reflective area on a non reflective surface. $49.00
| 15161 TE
Hand held photo reflective tachometer. 5 digit LCD display measures RPM from 5-99,999 with auto range select. Resolution: .1RPM