Build LED Headlights for a Longboard with a DC Motor

2017-06-14 | By All About Circuits

License: See Original Project

Ever found yourself coming home after a late night with only a longboard for transportation? Ever wished that your board had a headlight so you could actually see where you were going? Well, I haven't, but I know someone who has, so I made them a headlight for late night trips. The easiest way is to just hook up an LED to a battery with a switch, but how boring is that? Why not give our board a generator so we don't have to turn it on and off. How hard would that be? The answer is a little convoluted; the circuit, itself, is simple but attaching the generator is another story.

Parts List

Let’s start with the circuit. I used a permanent magnet DC motor for our generator. These are pretty common, and although they’re not optimized to produce a lot of power, we don't need much to power a couple of LEDs. Our generator (or motor, I use terms interchangeably throughout this article) will produce a voltage proportional to its rpm. It requires 5 V and 20 mA to run at 3600 rpm under no load. This won't be what my motor will produce when used as a generator, but this gives me a pretty good idea of what to expect from it. Since we only have one data point, I wasn’t sure how this would scale at higher rpms. This could present a problem because, when I calculated the rpm for the generator at 5 mph, we get an rpm of 7133. Longboarders going downhill can reach speeds of 60 to 65 mph, which calculates to 92737 rpm. We’d end up with 129 V and 515 mA if the voltage and current are perfectly linear with rpm. We’ll need to add protective elements in order to avoid such a high voltage.

I modeled my generator as a variable DC source. It can vary it from -10V to 10V. I also added a small resistor to separate the ideal voltage source from the rest of the circuit. I added one capacitor in parallel with the generator, this was to provide power stability for any momentary loss of power from the generator. Larger capacitors will have larger momentary power loss and take longer to power on the LEDs. I added two sets of LEDs (a headlight and tail light for safety) with current-limiting resistors, also in parallel with the generator.

We now have a generator, a headlight, and a tail light. Now we need to add some protection from the possible voltage. Since our generator isn’t an ideal voltage source, we can sink the extra current, which will limit the voltage. To do this, I used two Zener diodes in series: one forward bias, the other reverse bias. In forward bias, a Zener diode drops a voltage of about 700mV like other diodes. In a reverse bias, however, it will drop the specified Zener voltage (Vz). I picked a Zener diode with Vz = 4.7V. With both diodes in series, this has a voltage drop of approximately 5.4V. I chose a rather large Zener diode rated for a continuous current of 1A, which is probably excessive but I wanted to play it safe. This circuit will work if you aren’t using your longboard in reverse. If you’d like to use it in reverse, just add two more LEDs in parallel in reverse polarity. I made my headlights on a skateboard, so I made it reversible with a second set of LEDs shown in the second schematic.

Schematics for one-way and reversible generator designs

With the circuit done, now comes what was the hard part for me... attaching the generator. If you have experience with fabrication or if you already have something in mind to attach your generator, feel free to skip this part. If not, this is how I did mine.

There are a lot of ways to fabricate a mount for a generator, but I wanted to do something that college students living on campus with limited funds and supplies could obtain. I did this because I felt that the majority of longboard commuters that would be interested in this project are college students. I casted a mold with JB Weld steel epoxy, which was a new fabrication method for me. All you need is some paper and tape to do this. It’s easier if you use petroleum jelly (vaseline), wax, and a razor blade, but they aren’t necessary.

For this method, I used the paper and tape to make a form around the generator and the axle. If you use any wax, you’ll want to add some now to seal the edges and refine the shape of your forms. Coat any surfaces you don’t want covered with JB Weld with petroleum jelly, as well as any paper, tape, and wax. If you don’t want your generator to be permanently attached to your longboard, you’ll want to coat the axle as well. Then, tape the generator drive shaft to your wheel, mix up a bunch of JB Weld, 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 it has cured, you’ll need to remove the wheel and use some tape to pull and hold the generator drive shaft at a slight angle so it’s closer to where the wheel will be. After that, 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 your mold finishes curing, you can remove it and use the razor blade to clean up the edges. You should now have a formed mount with your generator already mounted; just pull the generator's drive shaft out of the way and put the wheel back on. The slight angle from the second JB Weld "pouring" is to keep constant tension even as the wheel slowly wears down.

Reattaching the wheel after the form is removed

Reattaching the wheel after the form is removed

Now that the generator is mounted, it's time to connect your circuit. Since the circuit is so simple, I just soldered the components’ leads to the wires and other components directly. After that, I used electrical tape to protect the solder joints and prevent shorts. I also used it to secure the LEDs and wires on the board but, if you prefer, JB Weld is an insulator and can be used to secure the LEDs and wires directly.

Red and white LEDs soldered to resistors and wires before adding electric tape

Finished board

Below is a video of me trying not to kill myself while I attempt to skateboard (first lessons can be pretty rough).

The methods used in this project can be adapted to work on most wheeled methods of transportation like bicycles, scooters, and roller blades. If you’re feeling adventurous, you could even adapt it to work on non-wheeled methods of transportation.

If you have made something similar, show us your project!

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Build LED Headlights for a Longboard with a DC Motor

2017-06-14 | By All About Circuits

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