Hello Electrical/Electronic guru's.

Please tailor your answers into simple non electrical terminology please. Most things mechanical engineering is my strong point, electronic/electric black magic is not !!!

Anyway, my problem. I have a little project which involves changing my current normal rear brake light setup of my enduro bike to a hopefully led setup. Normal bulbs do not last long off road !!
I have acquired, shall I say, a suitable 12V rear led light unit, but............its either on or off, if you see what I mean.

How can I get the unit to put out a dual light intensity such as a normal steup for example 5w/21W ?

Can I jam a resitor in sowmehere or somtheing to alter the intensity when the brakes are applied. How do I do this ? At the moment there are two cables coming from the led unit earth or 0V & 12V i'm assuming. The wiring on the bike has 3 cables, earth, 12V to the 5w filament in the bulb and 12v to the 21w filament.


I know its possible to buy off the shelf units for my needs for about £30 but as I've got something, I thought there might be a locost challenge somewhere along the line before I shell out.

Any suggestions or guidance greatly appricated. ?
Cheers

[Edited on 22/8/08 by clanger]

[Edited on 22/8/08 by clanger]

Use a put a resistor in series with the LEDs. Put a switch (either micro-switch of the brake leverl or a hydraulic pressure switch) in parallel with the resistor.
Finding the right resistor will be a bit of trial and error. You'll need quite a chunky resistor too.
I'd use pissy little resistors to find the right resistance then buy a big chunky fella for the actual bike. The wattage of the resistor will depend on how much juice you're removing from the 12v....

Dont forget you will need a pair of diodes!!!


Otherwise when you put the brakes on, the front light will come on as well

Consider the diode as a one way valve...

Simply connect the feed ot the 21W side of the bulb straight through the diode into the lights, and feed the 5W feed from the sidelights through a resistor into a diode into the lights....

Without knowing exactly what LEDs you are using I cant suggest a resistor...

But when I build conversion kits I usually use 1/4W resistors of around 200R (ohms)....but it might be a bit of trial and error on your part!

[Edited on 22/8/08 by tegwin]
Rescued attachment bike light.JPG

^^^ yep; that circuit wold work - good point about the side lamps.

youo don't need the diode in wire from the brake.

I wold suggest starting with a 10 ohm 2 W resistor and then increasing value if bulb is too bright; decrease if too dim.

alternativey fit diodes in series instead of the resistor - these drop about 0.7 V - about 4 to 6 of these in total would be about right.; some linke 1N4006

You are right, you dont NEED a diode on the brake feed...

But for piece of mind, I think I would anyway!

Brilliant, thanks boys. Even an elec knob like myself can do that......................I hope

Or you could just buy a standard LED replacement for the dual filament 21/5 Watt lamp. They are very cheap now, e.g. ebay clicky