Topic: electrically gifted people please read this ~

[Raul CB750K1]

From what I understand, the LED unit is a high intensity one. Negative goes to ground, and positive goes to brake switch cable. You actuate the brake, the LED lits.

If you want to use the same LED unit to work as a regular taillight, you are right. You will need to connect a resistor between the taillight wire (+12V) and the LED (+), that by now should be already connected to the brake switch cable. In regular conditions, +12V will feed the LED unit through the resistor, therefore the power will be reduced. When you actuate the brake, +12V will be fed directly to the LED unit, glowing at maximum power. Then the resistor would be connected between 2 positives, being that absolutely harmless.


I have a friend with an old bike, 6V electrics. The headlamp bulb is expensive and difficult to find. Over here it is mandatory to ride with the low beam at all times, and it is bad for the bulb because it reduces its life. What my friend has done is to connect two big diodes in series with the bulb, but only in "parking" position. He rides at daytime in "parking", the low beam is energized throug the diodes, where 1.2V are robbed from the bulb, decreasing a little its power but increasing its life. At nightime he needs all the light he can afford, so he switches to "low beam" where the +12V is fed directly to the bulb. It is more or less the same than with the taillight, but using diodes instead of resistors.

[Gorms]

To achieve the "dual brightness", led lights use a kind of chopper circuit that makes the eye perceive the light as dim/bright.  They give the led's a square wave input that is perhaps 50% duty cycle for a "dim" and full on for "bright".  The duty cycle determines the "brightness" that the eye sees.  That's why (I don't know if anyone else has noticed this) you see some of the newer cars that have led tail lights, and when you quickly glance away from the light, you see multiple images of the tail light. like a very fast flashing strobe.
In the pic below, I used an LED truck tail light that has such a circuit built in to give you the strobing effect, and goes full on when you hit the brake.

Thanks for jogging my brain Steve.  I forgot about the use of duty cycle.

[Industrial Cafe]

yeah me too.
you said "doodie" cycle.

[cycleman]

In a normal tailight set up on these bikes they have a two filament bulb, one for the tail/running light & the other for the brake light.  Some older hard bags like the Wixom had two separtate bulbs one for each of the above that you could wire into the circuit to improve visibility.

From what I've seen/understand of the LED tailights, some have the provision for two different circuits, including a licence plate light.  They basically start off with about 20 or so LED's and then divide them up so that ie: 4 are for licence, 6 for running & 10 for brake.   There are three wires, one for ground, one for running/taillight, one for brake. I'm pretty sure you don't have to do any special wiring with these, but you do if you are trying to use LED's as signal lights.

I'm thinking of doing something similar and have been doing some searching on ebay, etc. & there are some fairly inexpensive setups out there.  Just haven't figured which way to go.

[78whiteorbs]

Raul CB750K1 -  I tried this and maybe I am using the wrong reisitor or something but it isn't reducing it at all the light is the same brightness. I think I am gonn a just wire four more LEDS to the brake light and install them for when the brakes are pressed i will get 10 LEDS glowing instead of six for the running lights. I am assuming I can just run the leds in a series between the neg and positive of the brake wire circuit. 4 LEDS are cheap at radio shak , I will post pictures as to how this turns out . This is the most I have had to use my head in this project so far....bah!  The bike I have has permanantly on running lights/head light so I want to install an on off switch for incase I want them off. This will be interesting and I hope it might help someone save some dough on things that can't be modified or DIY .

[bmarshall]

Mine is a simple 12v bulb w/ 3 wires; running light and brake light, works fine for me.

[Laminar]

Standard 4" LED trailer light with built-in circuit for taillight and brake light. I wired it straight up to the stock tail light wires.

[skamagedon]

67 ohm resistor for every 5 leds for 12 v hooked upon the ground side. leds are generaly 2.2v forward voltage @ 20 mah

[phil71]

not touching this thread with a 10 foot pole. So much wrong info I don't know where to begin.
  I will say only this: DO NOT take advice about what resistor to use by anyone who spells it "resister".

[Bodi]

An LED produces light proportional to the current through it. The voltage across the LED is relatively constant regardless of current.
Again, the light output is proportional to the current: more current equals more light. An LED is completely "dimmable" from zero light to maximum light. The maximum continuous light output is at the rated current for the particular LED. The maximum current is based on thermal properties of the LED, it produces heat when passing current and will overheat and fail if operated continuously above the rated current.
There's a trick, though. If you pulse the LED at a higher current and have the duty cycle of the pulses arranged so the total average power is below the rated power, it looks brighter but doesn't burn out. This can also make a LED look as bright as at rated DC current with less actual power.
Anyway, most trailer lamps have 3 terminals - ground, tail, stop. The current limiting resistors for 12V automotive power are inside, one set for dim "tail" light and one for bright "stop" light.
I don't think there are any trailer tail lamps at your local auto parts store that are pulse driven. The electronics required is much more expensive than a few resistors.

[Gaither]

FWIW, I bought 2 small LED lights (red) with chrome bases. They are about 2 1/2" long and 3/8" wide. I don't know how/why it works but they "burn" very visibly normally for running  lights and also very brightly for brake lights. Both modes use the same LED "bulbs" and appear to have 5 LED "bulbs" in each (enclosed) light.