Extreme Circuits

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LED Bike Light Circuit Project

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:

LED Bike Light Circuit Diagram

LED Bike Light 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.

Tags: led, bicycle, Light

Comments on this post:

David Ingram
By David Ingram (0) 0Score: 

1 year ago:  Why do cyclists want to flash their lights at 5 Hz, a frequency known to trigger epileptic fits.
Steady lights are sufficient, particularly when high brightness LEDs.

Jimmy McDonald

1 year ago:  "Why do cyclists want to flash their lights at 5 Hz, a frequency known to trigger epileptic fits."

First, 5 Hz is the very outside edge of the range most likely to cause seizures, so it's actually a very good choice from that perspective. (source: http://www.epilepsyfoundation.org/aboutepilepsy/seizures/photosensitivity/)

Secondly, flashing lights are very good at gaining attention -- something bicyclist struggle to achieve. It's very common for a steady bike light to be confused as a reflection when a driver is making a quick visual check.

I know your comment doesn't say that steady lights are equally attention getting as flashing lights, however, I do disagree with the notion that steady lights are sufficient in all cases. I know that as a driver I actually appreciate the flashing red lights on the back of bikes. I don't want to hit them and they often seem to come out of nowhere, no fault of their own.

alan Gibson
By alan Gibson (0) 0Score: 

1 year ago:  The steady lights will not be seen as easy.
I have driven right past people, not realizing some one was there.
The steady lights blend into much.
The easiest lights that i saw were blinking lights.
Then moving side to side and up and down or in circles lights are 2nd best.
The steady lights came in next place.
The last place honors or shame are those with out any lights.

So if you want to stay alive, go with the blinking lights on your bike. Not just front and back, but the two sides also. This from driving bus routes before sun rise.

David Bell
By David Bell (0) 0Score: 

1 year ago:  Even better than a single flashing light to attract attention is two (or more) lights than simulate motion. The eye/brain system is very sensitive to moving objects in the peripheral vision. Two lights in front, flashing alternately, and another two in the rear. It would double the components in the multivibrator, but add a lot in terms of visibility.

Anodic Zinc
By Anodic Zinc (0) 0Score: 

1 year ago:  Nice job using a simple circuit (always remember to KISS: Keep It Simple, Stupid!) As for spatial separation (two lamps side-by-side to give a shifting motion) just power the lamp/s on one side of the bike from one of the transistors, and the lamps on the other side of the bike from the other transistor. Only drawback would be that at a distance they will blend together and not blink, but if they're (autos) that far back they aren't a threat to your safety. And blinking lights are far more effective at attracting motorist's attention.

Adam Jackson
By Adam Jackson (0) 0Score: 

1 year ago:  That sounds interesting. The circuit project mentioned in the blog should work in every kind of design. What intrigued me more was the idea that by installing a new wheel with inbuilt dynamo, a stable power supply can be generated. I will try it this weekend. Hopefully it works for me as well.

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