During my mountain bike adventures, I used to rely on the familiar flashing LED lights available at retail stores. However, these lights often caused issues such as flat batteries and falling off during rides. Being an electronics student, I thought there had to be a better solution. To improve my setup, I initially purchased a front wheel equipped with a built-in dynamo in the hub, producing a smooth sine wave of 30 Vpp (at no load). Utilizing this knowledge, I crafted a simple power supply. The transistors I used were BD911, which might be considered overkill, but they were readily available at my school, hence my choice. Smaller alternatives would suffice as well. The power supply is connected to an astable multivibrator, alternatingly powering the front and rear lights.
The frequency is determined by the RC time-constant of R3 and C3, and R2 and C4. This time can be calculated with the formula:
t = R3×C3 = 20×103×10×10-6 = 0.2 s
You can use a 22 kΩ (common value) for R2 and R3, that doesn’t make much difference. On a small piece of prototyping board are six LEDs with a voltage dropping resistor in series with each pair of LEDs. Such a PCB is used for both the front and the rear of the bike. Of course, you use white LEDs fo the front and red ones for the rear.
The PCB with the main circuit is mounted under the seat, where it is safe and has been working for more than a year now. There are a few things I would change for the next revision. An on/off switch would be nice. And if the whole circuit was built with SMD parts it could be mounted near the front light. This would also be more convenient when routing the wiring. Now the cable from the dynamo goes all the way to the seat and from there to the front and rear lights.