To use the Revolution, Reactor, or Beacon, you need to connect it to a power source. If you are going to create your own power from the bike, you'll want a dynamo (also called a generator). There are two common types of dynamos:
A Hub dynamo takes the place of the hub in your front wheel. They are powerful and efficient, but more expensive. You can't add just a hub dynamo to an existing bike - you need to either buy a wheel built with a hub dynamo, or buy a hub dynamo and have your bike shop use it to build a wheel.
A bottle dynamo generates power as it spins against the side of your tire. They are relatively inexpensive and can easily be installed on an existing bike. If not available at your local bike shop, searching Google or eBay for "Bicycle Bottle Generator" or "Bicycle Bottle Dynamo" will give good results. You can probably find a bottle generator for under $20.
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