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LED Bike Light Circuit(BD911)

2014-12-16 21:26
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This article briefly describes the LED Bike Light Circuit (BD911). This principle is easy to understand, but also very practical. Depth understanding of circuit elements, can be more effective to grasp this principle. Do you know the circuit, for example, can understand and buy these components: BD911 .

On my mountain bike I always used to have one of those well-known flashingLEDlights from the high street shop. These often gave me trouble with flat batteries and lights that fell off. As an electronics student I thought: “this can be done better”. First I bought another front wheel, one which has a dynamo already built in the hub. This supplied a nice sine wave of 30 Vpp (at no load). With this knowledge I designed a simple power supply. The transistors that are used are type BD911.

These are a bit of an over-kill, but there were plenty of these at my school, so that is why I used them. Something a little smaller will also work. The power supply is connected to an astable multi-vibrator. This alternately drives the front light and the rear light. 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 22k (common value) for R2 and R3, that doesn’t make much difference. On a small piece of prototyping board are sixLEDs with a voltage dropping resistor in series with each pair ofLEDs.

Circuit diagram:

Figure 1 LEDBike Light Circuit Diagram

Such aPCBis used for both the front and the rear of the bike. Of course, you use whiteLEDs for the front and red ones for the rear. ThePCBwith 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 withSMDparts 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.