An old bicycle light generator usually refers to a bicycle dynamo—a small device that generates electricity from the motion of the bike to power lights. 
What it is
The classic type is the bottle dynamo. It’s mounted on the frame so a small roller presses against the bicycle tire.
How it works
- When you pedal, the wheel rotates.
- The roller on the dynamo spins against the tire.
- Inside the dynamo, a magnet rotates in a coil (based on the principle of electromagnetic induction).
- This produces electric current that powers the bike’s lights.
Common types of old generators
- Bottle Dynamo – mounted on the frame and touches the tire.
- Hub Dynamo – built inside the wheel hub (more modern but still mechanical).
Pros of the old dynamo lights
- No batteries needed
- Works whenever the bike moves
- Durable and simple
Cons
- Light becomes dim when riding slowly
- Adds a little resistance while pedaling
- Can slip in rain if it touches the tire
These generators were very common on bikes in Europe and older bicycles around the world.
If you want, I can also show you pictures of old bicycle dynamos or how to connect one to modern LED lights.