Use a DC Motor to Power LED Headlights

Build a generator-powered LED headlight for a longboard for very cheap.

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)

Generator

Red LED

White LED

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 tim