ok, I know this is possible.....somehow. I have 2 round LED lights I want to use as taillights, brake lights, and turn signals. problem is they are 2 wire lights, one power, one ground. I know there is a way to use 2 wire ones as brake and tail using resistors (not sure which ones I need) on the tail wire to reduce voltage and dim the light when using as taillight only, then when the brake is applied it applies full voltage to the light for the brake to get brighter, but how would I wire the turns into them too. I know it would require using diodes so the voltage doesn't bleed back, but how would I wire it to make it flash even when the brakes are applied? the lightsi want to use are round LED marker lights from a semi used as marker lights.

Using resistors to dim the lights won't work with LEDs, they are either on or off.

QuoteUsing resistors to dim the lights won't work with LEDs, they are either on or off.

I hate to disagree but that is not exactly true, if your incoming voltage is around threshold (2-2.5Vdc) yes it is a either an on or off condition, but above threshold it causes the LED to burn brighter. By adding resistors in series you are causing a reduced voltage to the led and that will dim the light emitted. Where you can run into problems dong this is that if the resistor is not heavy enough (wattage) it could heat up and break down resulting in the voltage passing through or worse melting the wiring around it and leading to some very exciting moments as you ride down the highway.

I added side marker LEDs to my ZG1000 as auxiliary tail/brake lights. what you need to do is solder a resistor and a wire lead to the end of the positive lead of your LED, the resistive wire goes to the taillight connection and the straight wire goes to the to the brake lead. That way when just running tail lights there is reduced voltage to the LED resulting in less light, when the brake is used the LED gets full voltage and will burn brighter. Keep in mind you need LEDs that will operate with voltage > 12Vdc  as the charging system will be putting out 13 or 14 volts.

what kind or size of resistor? 

Quotewhat kind or size of resistor?

I think I used 1/2 watt but don't remember how many ohm.  I had several 1/4 watt resisters and just started checking to see what gave me the best contrast in light, once I had an approximate total resistance I went and picked the 1/2 watt that I needed.

If you are adding multiple resistors to create a larger value remember that in series (end to end) - - - - - -   the resistance is cumulative. In parallel (side by side) I I I  the resistance is reduced.   Something else I just remembered, I think I put a diode in each leg just so there would be no issue with flyback voltage. I don't think that they are absolutely necessary but I'm a belt and suspenders type guy (no wise cracks from the Brits!! ) and they are cheap insurance. With a diode the voltage flow is toward the line     pos ->  ---]-  ---> light

Quote from: sblack on June 23, 2009, 12:17:39 AM
Using resistors to dim the lights won't work with LEDs, they are either on or off.

Not true.  There are modules you can purchase to convert on/off LED turn signals to on/off/running signals...  they will work for taillights as well.

The easy way, as An said, buy a module (probably designed for trailer) and rework it for 2-wire.  And that doesn't sound worthwhile to me, and my background is in electronics.  If you REALLY want to know the resistor needed, give us some more info on the LEDs.

yeah I was totally wrong. never saw lights lit up and assumed they were LED's. I "sacrificed" one and cut it open and it is just a single filament bulb. my goal was to run r leads off the power wire, one with resistor to  dim it a little to use as running light, and 1 full power for the brake light. then I want to get the trailer 3 into 2 convertor and make them my turn signals too. I have an unlimited supply of these lights, I drive a yard truck at a factory where every semi that comes in has a half dozen or so on these!  I read on a chopper forum how to do it somewhere and now I can't find it again. I am doing this on my GS, and have been eyeing up some lights at a truck stop that I want to use on my chopper/bobber project and they are single filament too.
so it is just a basic single filament bulb that I need to dim down a little and need a starting point for the resistors?! 

So you want to go ahead with the filament bulbs?  What's the "wattage"?