Description
On my mountain bike I always used to have one of those well-known
flashing LED lights 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 six LEDs with a voltage dropping resistor in series with each pair
of LEDs.
Circuit Diagram:
Such a PCB is used for both the front and the rear of the bike. Of
course, you use white LEDs for 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.
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