Nice build, worth doing if you are playing with motors and need to figure out how fast they are spinning.
http://www.instructables.com/id/Measure-RPM-DIY-Portable-Digital-Tachometer/
Nice build, worth doing if you are playing with motors and need to figure out how fast they are spinning.
http://www.instructables.com/id/Measure-RPM-DIY-Portable-Digital-Tachometer/