Electronics guru required - dull down my LED tail lights

Are you an electronics guru ? Bored at home and would love to put ohms law to the test ? Know more about electronics than Dick Smith ? OK, read on

I put a set of LED tail / stop light globes (I got the clear ones, not red, but jaycar only seem to have the red ones in their cattledog ATM) into my 9r (and put a set in Cath's 6r too). The lights work just fine, but there is not (IMnot_soHO) sufficient differentiation between tail lights and brake lights ... in other words, the tail lights are too bright.

Now I remember enough electronics to know (well think I know) that a resistor applied to the tail-light circuit should serve to dim the bulbs, but, without sitting down with a box full of resistors and experimenting, I have no idea what resistance resistor (what a mouthful that is !) I should be starting with.

So come on genius, what resistance resistor should I start with, and why ?

In all seriousness, if you have done this or know someone who has you could save me a heap of wasted time and skinned knuckles (don't ask, I always skin my knuckles when working with electronics )

Thanks,

try a 100k resistor, 10% tolerance (Brown band?), 10W if you can find one ( a biggie), or 5W minimum.......the current draw of the LED's shouldn't be very high....so 5W will *probably* be OK.....

see how that goes.......100K should be *fine* to reduce brightness enough between stop / tailight.

Buy a potentiometer (is that what's it's called in english?) ... that knob on old stereo volume, but the small one. Buy 150kOhms and you can play around using small screwdriver.

Thanks Gos,

I assume "100k" is a 100 kilo ohm (or Kohm).

Java, it is a potentiometer (or variable resistor) in english, well done ! The reason I'd rather avoid using one is that I then have another weak point in the taillight circuit (vulnerable to moisture / dust / etc). Sure I could use it to determine the necessary resistance then replace it with a solid-state resistor, but I'm trying to avoid experimenting (skinned knuckles and all that, normally from dragging them along the ground in an ape-like fashion )

Thanks though

Just go an google it! Do a quick 1/2 hour electronics course and you will figure it out and know the proceedure for next time.

I think what you want is the power law here.

Ohms = (volts^2/ watts)

So if ya have a 12V bike, and the leds are collectively rated at say 2 watts you have a 72 ohm resistance in the circuit. If you want them dimmer, running at say 1.5 watts, you will need a resistance of 96 ohms, or a 24 ohm resister added in series to the circuit. But make sure it's large enough to handle the current. A 2watt resistor, not a 1/2 watt one.

http://www.the12volt.com/ohm/ohmslaw.asp

Go low tech.... get a small hammer and smash every second bulb...

Free advice from Stereo ...

Go low tech.... get a small hammer and smash every second bulb...

Free advice from Stereo ...

Too labor intensive! Just stuff wads of red cellophane in the tail-light assembly

I think austreetfighter sell a special thinymajig for this problem

I had some Led 1157 replacements and a clear lens and though there wasn't enough difference between tail lights and brake lights, but it wasn't because the tail lights were to bright, it was because the brake lights were too dull. (Although there are less LED's than the Jaycar ones)

1157 Led replacements


An animation of brakes on and brakes off using LED's


An animation of standard 1157 globes.

I'm pretty sure that you need to maintain the voltage to the LED any vary the current.

Most simple resistors will cause a voltage drop to the rest of the circuit (hence dimming most ordinary lights) however with LEDs this doesn't work so well. A better solution would be to reduce the current to the LED whilst maintaing the voltage.

I'm pretty sure you need some kind of transistor/resistor combination (to switch the current on and off very quickly) to achieve this. Might have to ask someone with more knowledge than me tho

i put a LED globe in, and gave up for the very same reason, just too freaking dull. i put them in the car and same result, too freaking dull (this was the white leds, not the red ones, that may make a difference, or they were just shit intensity globes....)

dave#3,

What are the LED rated at? in watts.
What is the original tail light rated at?

I'm not sure if it's as simple as adding a resistor to it. You gotta look at the circuit diagram to see if it's 'sharing'(series or parallel) the same line with other devices.

I'm not an automotive electrician, but I do know abit about electrical.

I got a small led torch, one of the blue light keyring ones. Anyway it takes 4 tiny batts, but when I reloaded it I only had a much smaller set and 5 or them were needed to fill the tube. It's brighter now, so a simple voltage change does effect them. You would have to put it "south" of any circuitry though.

This thread is atarting to get confusing

I wish I had never posted on it now...