Have you ever found yourself coming home late at night with only your longboard/skateboard for transportation and wished that you had a headlight so you could actually see where you were going? I haven't, but I know someone who has, and they thought it would be great if I could build them a headlight for late night trips. The simple way would be to just hook up an LED to a battery with a switch, but how boring is that? Why not put a generator on it so we don't have to bother with turning it on and off. Well, how hard would that be? The answer is a little convoluted; the circuit is pretty simple, but attaching the generator is another matter.
Parts List (Only one direction)
Capacitor( 4.7 uF)
100 ohm resistor
330 ohm resistor
Two 1N5337BG Zener Diodes
I'm going to start with the circuit. We are going to use a permanent magnet DC motor for our generator. These are pretty common, and although they are not optimized to produce a lot of power, we don't need much power to run a couple of LEDs. Our generator (or motor, I will be using the terms interchangeably throughout this article) will produce a voltage proportional to the rpm of the generator. The DC motor that I will be using was salvaged from a burned out cd drive. I lost the model number but wrote down the motor spec. Under no load, it requires 5 V and 20 mA to run at 3600 rpm. This won't be what my motor will produce when used as a generator, but gives me a pretty good idea of what to expect from it. But with only one data point, I don't know how this will scale at a higher rpm. This could present a problem because when I calculated the rpm for the generator at 5 mph, I came up with the number 7133 rpm. Longboarders going downhill can reach speeds of 60 to 65 mph, giving us 92737 rpm. If the voltage and current are perfectly linear with rpm, we end up with 129 V and 515 mA. We will need to add protective elements to avoid such a high voltage.
I am going to model my generator as a variable DC source, it can vary it from -10V to 10V. I will also add a small resistor (R5 and R10) in series to separate the ideal voltage source from the rest of my circuit; this will make the simulation workable. I'm going to add one capacitor in parallel with the generator, this will provide just a little power stability for any momentary loss of power from the generator. The larger the capacitor, the longer the momentary loss can be, but it will also take longer to power on the LEDs. I will then add two sets of LEDs (headlight and taillight for added safety) with current limiting resistors, also in parallel with the generator.
We have a generator, a headlight, and a tail light. This seems pretty good, but we have not added any protection from the possible voltage. Because our generator is not an ideal voltage source, we can just sink the extra current, thereby limiting the voltage. To do this, I will use two zener diodes in series, one forward bias, the other reverse bias. When a zener diode is used in forward bias, it drops a voltage of about 700mV like other diodes. However, when one is used in a reverse bias, it will drop the specified zener voltage(Vz). I selected a zener diode with Vz = 4.7V, with both diodes in series, this gives me a voltage drop of approximately 5.4V. I selected a rather large zener diode that is rated for a continuous current of 1A, this should be excessive, but I wanted to play it safe. This circuit is suitable if you are never going to use your longboard in reverse. If you want to use it reverse, just add another two LEDs in parallel, in reverse polarity. I'm doing this on a skateboard, so I made it reversible with a second set of LEDs represented in the second schematic.
Single Set of LEDs, LED1 is red, and LED3 is white
Schematic for reversible generator design
With the matter of the circuit done, now comes what is probably the much more challenging part, attaching the generator. If you have experience fabricating or have something in mind, feel free to skip this part. If not, this is only one of many possible methods to fabricate the mount for the generator.
There are more ways to fabricate a mount for the generator than I can count. I wanted to do something that doesn't need any tools beyond what a college student might have in their dorm room and supplies that could be ordered and used for just a few dollars. I did this because I felt the primary longboard commuters that are interested in this project are college students. I decided to experiment with a new fabrication method for me. Casting a mold with JB Weld steel epoxy. In order to do this, all you need is some paper and tape. It is easier with petroleum jelly(vaseline), wax, and a razor blade, but these are not necessary.
For this method, you are going to use the paper and tape to make a form around the "axle" and generator. If you are going to use any wax, you will want to add some now to refine the shape of your forms and seal the edges. With the petroleum jelly, coat any surfaces you don’t want covered with JB Weld, as well as any paper, tape, and wax. If you don’t want to permanently attach the generator to your long board, you will want to coat the axle as well. Then, tape the generator drive shaft to your wheel, mix up a bunch of JB Weld with disposable tools and add it to your form. You don’t want to completely fill your form at this time.
First mold with generator taped to wheel
Let the JB Weld cure overnight. Once the JB Weld has cured, you will need to remove the wheel and use some tape to pull and hold the generator drive shaft at a slight angle, closer to where the wheel will be. Mix up another batch of JB Weld and finish filling your form.
Filling the form the second time. Wheel Is taped to axle under light tension
After it has finished curing, you can remove your form and use the razor blade to clean up the edges. You should now have a nicely formed mount with the generator already mounted; just pull the generator's drive shaft out of the way and put your wheel back on. The slight angle from the second JB Weld "pouring" should keep constant tension even as the wheel slowly wears down.
Form removed and wheel reattached. You can see the generator with the drive shaft.
With the generator mounted, it's time to connect your circuit. Because the circuit is so simple, I just soldered the leads of the components to the wires and other components directly. That was followed by Electric tape to prevent shorts and protect the solder joints. I also used Electric tape to secure my wires and LEDs to the board, but if preferred, JB Weld is an insulator and can be used to directly secure the LEDs and wires.
Red and White LED soldered to resistors and wires before electric tape.
Below is a video of me trying not to kill myself while I skateboard (first lessons can be pretty rough) to demonstrate that it works, sorry about how dark it is.
The methods used for this project can be adapted to work on most wheeled methods of transportation, including bicycles, tricycles, roller blades, scooters, and unicycles. With more work, you could even adapt it to work on non-wheeled methods of transportation.
If you have made something similar, let us see your project!