Putting a LED instead of an incandescent bulb on my bike


So a few days ago my rear light burned up. It’s not that it’s very important, what’s essential  is the front light during a night. Since there’s that reflective red plastic on the back, I wouldn’t worry. Anyway, I decided to replace the burned bulb.

But I felt bad about placing in its place another bulb, why not get a little creative?

So I decided I should put an LED instead. A problem appears: the bicycle’s power source is a dynamo. That means alternating current. LEDs still work under these conditions, although at low speeds they’ll pulsate instead of displaying a continuous light. Anyway, I kind of worry that it will wear out quicker because of the stress it has to handle when the current reverses. In these conditions, I had to also make a bridge rectifier. To smooth out the voltage, I also added an electrolytic capacitor( actually I added 2).

I tried measuring the output voltage of the dynamo on the last gear, and I was confused when I saw no reading. For 10 minutes I tried to figure out if there was a broken wire or some loose connection. Then I noticed I was trying to measure alternating current using the DC position on the multimeter :). Facepalmed, and then proceeded to measure again. About 10 volts.

The LED I used is a bright flux LED( the square type), and red LEDs work at around 2V and 20mA. Using Ohm’s law, it means that I need a R=(10-2)/0.02=400 Ω resistor. I didn’t have a resistor with that value around, so I added a 300Ω and 91Ω in series.

For easier connection, I broke the old light bulb’s glass and used its cap to which I soldered some wires.

Here’s the circuit schematic( I made it using a free little program named TinyCAD):I felt well after finishing this, but I’m not very satisfied with it. First of all, the LED is not as bright as I expected. I’ll have to buy a high power LED and try to experiment with that. Then I’ll also want to add a more powerful capacitor, I want that “fade” effect to happen when stopping the bike. Lastly, I need to make the whole construction sturdier. Right now, if I shake the bike really well it’s not sure if the circuit will continue to work or some connection will loosen up.

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