How does a vibrating cell phone or pager work?

If you have a cell phone or a pager, then you know that
having it ring in the middle of a movie or performance is
enoughto get you killed in some cities. Vibrating devices
that quietly replace the ringer are therefore life-saving
devices that are an important part of urban survival!

There is, however, a device that takes vibration to high-tech
extremes. Any parent whose child owns a Tickle-Me-Elmo doll
has experienced this technology. Elmo has a vibration system
(designed to simulate body-shaking laughter) that is powerful
enough to cause many children to drop the toy. The vibration
system inside a pager works exactly the same way on a smaller
scale, so let's use Elmo as an example.

Here is a Tickle-Me-Elmo doll:
Minor surgery reveals the control unit:
Inside the control unit (on the right hand side in the above
image) is a small DC motor which drives this gear:

You can see that, attached to the gear, there is a small
weight. This weight is about the size of a stack of 5 U.S.
nickels, and it is mounted off-center on the gear. When the
motor spins the gear/weight combination (at 100 to 150 RPM),
the off-center mounting causes a strong vibration. Inside
a cell phone or pager there is the same sort of mechanism in
a much smaller version.