Today I would like to show you how to build a very cheap yet working bicycle generator. My idea is not competing with a store-bought “dynamo”, but instead teaching you a fun project made from trash (and some of the science behind it).
Step 1: What’s a Generator and How It Works?
A generator is a machine that converts movement into electrical power using a principle called “Electromagnetic Induction”.
Every time when electrical current travels through a conductor (let's say a piece of wire), a magnetic field forms around that conductor. You may ask “If that’s true, why my cellphone charger cable isn't sticking to metallic stuff right now? Well, the strength of the magnetic field is proportional to how much current is flowing through the conductor, and in the case of your charger the current is negligible. But with more powerful currents the field is significant enough to, for example, lifting industrial metal scraps with a crane.
Electric field running through a conductor. Result: Magnetic field is produced around the conductor
Step 2: What’s a Generator and How It Works? (cont.)
The principle also works backwards. If you move a magnetic field near a conductor, an electrical current will be induced in the conductor. Now, by “moving” it’s equally valid to physically move the magnet or just turning the magnetic field on and off repetitively.
Moving a magnet near a conductor. Result: Electric current is produced on the conductor
Step 3: Real World Generator Example
No matter if you buy a old squeaky “Bottle Generator” like the one on the attached picture, or a "hub generator" (the beautiful noiseless alternative), the principle is the same. The movement of the spinning wheel spins a magnet, which is in close proximity with a wrapped coil of wire.
The interesting thing is that a toy motor can also be used as a generator. The motor converts electricity into magnetism, and magnetism into movement, right? Well, spinning the motor will produce electricity. Same principle, used backwards: Moving the magnets inside the motor induce electrical current.
Step 4: Enough Theory, Let's Make the Generator Now!
As you obviously guessed, we will need a small electric motor. I got my motor from a burnt hair dryer. You can get yours from an old toy, it doesn't really matter if the device is broken as long as the motor still works.
Step 5: Preparation
This motor has a couple of ventilation openings on the sides. This is for refrigeration, but I'm closing them to avoid dirt and crap from the road entering the internal parts.
Edit: Don't try to re-use motors from computer fans because they use a slightly different working principle. The same applies to step Motors from old printers.
Step 6: The Poor Wheel
To transmit the rotation between the wheel and the motor I'm going to use a rubber wheel from a LEGO set I stole from my friend Juan when we where kids. Sorry pal, I'll someday return it.
Note: I strongly recommend you to find a creative alternative to the wheel. The truth is, a LEGO wheel is not designed to withstand the loads of a bicycle ride and will fail soon.
Step 7: Adapting the Wheel to the Motor Shaft
You can attach the wheel using super-glue
...but I just liked to show off my new set of drill bits, so I inserted a retaining screw, taking advantage of a flat face on the motor shaft.
Meh, this was a total overkill for a wheel that will not last long. So forget it, just attach the wheel with your favourite super glue!
Step 8: "Spring" System
The bicycle rims tend to be non-aligned, and you must provide a method to maintain a constant pressure between the generator wheel and the rim no matter of the bumps or depressions.
A commercial generator is mounted on a spring loaded structure, but since we are working with dirt here, a piece of foam will do. Obviously you are free to find a more decent method after you finish the experiment.
Step 9: Fastening the Assembly
Since this is a somewhat disposable arrangement, I used zip ties. Find a balanced point where the motor doesn't fall from your bicycle, but there's enough pressure from the foam to keep the LEGO wheel spinning against the rim. Don't get frustrated if you don't nail it at the first time.
23.2 GIGAWATTS!!! 23.2 GIGAWATTS!
Nah, is really is 12 volts under load. But the current is decent enough to light a small bulb from a car interior light.
Step 11: Slight Improvements
You can focus the light using parts from a flashlight
Step 12: No LED for Now, But Coming Soon!
You can see I used a small 12V, very inefficient, incandescent bulb.
After building this prototype the next natural step would be wiring it up your much more efficient and good looking existing LED lights.
But I recommend you to wait, because some considerations must be taken. For example, depending of the type of motor you pick up for the experiment, your generator will end up producing DC or AC current. You should also provide a way to regulate how much current goes to your LEDs.
Step 13: DC Vs. AC Current and Voltage Regulation
If you see a battery, the terminals are labelled with + and -, and this arrangement is maintained during the life of the battery. This is called DC, or Direct Current. But other power sources, like the outlets at your home, provide current that is constantly changing sides (technically called "polarity") each second.
AC or Alternating current is very useful for power transmission, but unfortunately your LEDs will not like it. Since they only glow if polarized on the correct way, they will blink like a party strobe light every time polarity is reversed. They can withstand this condition for some time, but inevitably they will burn out.
Even If you are lucky and your generator produces DC, there's another problem. The output of your generator will vary proportionally to the wheel speed. That means that if you go too fast, the voltage output will also sky rocket. Brightness of your LED will increase until again, they inevitably burn out.
But fear not: we can fix all of this using some cheap electronic components on a next instructable.