How does a vibrating cell phone or pager work? | HowStuffWorks
This weight is about the size of a stack of 5 nickels, and it is mounted offcenter on the gear. When the motor spins the gear/weight combination (at 100 to 150 RPM), the offcenter mounting causes a strong vibration. Inside a cell phone or pager there is the same sort of mechanism in a much smaller version. Advertisement.