How rear lights work, brake/parking. cheap LED idea in the making.

JJ.Foulds

JJ.Foulds

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So im looking at my rear lights, they are just the normal bulb ones and i have 2 bulbs either side which act as both normal AND the break lights.

So when i brake, the lights get brighter, now im guessing this is because the voltage is increased from 6v? to the normal 12v to make the brightness change.

Now my thought is, because im a cheap guy, i could wire/soldier in some led strips (either red or white not sure yet) stuck inside the housing with a resistor(so no bulb out errors) for like under a tenner easily, and they would still get brighter and dim the same.

Im just putting this idea across is electronically this would work, im guessing it would? and maybe if someone can point me to the correct resistor i would need would be great for my retro fitting ;)

Cheers.
 
Yours must be a different year or maybe a 5 door. Mine has separate bulbs for each function.

Assuming what you say is correct (and I know Vauxhall used to do it on Vectras), it's not as simple as 6v/12v - it's probably Pulse width modulation (it is on Vectras) which works by effectively switching the bulbs on and off very fast (so fast you don't notice) by varying the "on" and "off" times, you get the greater brightness. Some LED circuits use this too, but it needs to be designed with Leds in mind, or else the effect is flickery because leds switch on and off much more quickly.

I dunno if it would work, but I tend to doubt it. BTW the general word on t'internet seems to be use red Leds even if behind a red lens - not sure why, something to do with how they produce colour means white led behind red lens doesn't work as well apparently.
 
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JohnE said:
Yours must be a different year or maybe a 5 door. Mine has separate bulbs for each function.

Assuming what you say is correct (and I know Vauxhall used to do it on Vectras), it's not as simple as 6v/12v - it's probably Pulse width modulation (it is on Vectras) which works by effectively switching the bulbs on and off very fast (so fast you don't notice) by varying the "on" and "off" times, you get the greater brightness. Some LED circuits use this too, but it needs to be designed with Leds in mind, or else the effect is flickery because leds switch on and off much more quickly.

I dunno if it would work, but I tend to doubt it. BTW the general word on t'internet seems to be use red Leds even if behind a red lens - not sure why, something to do with how they produce colour means white led behind red lens doesn't work as well apparently.
Click to expand...

ahh, see my electrical knowledge is limited but enough for some things.

Iv wired in LED's into my interior lights, and made my own DRL which come on when car starts. The interior ones work perfectly, might this mean the rear would also work, or do you think it wouldn't with what you said before?
Or should i just wire them up and do a test?
 
The bulbs used for braking are 12v, but have double filaments, one is 5watt (constantly lit) , the other is 21watt (lit when braking(, when you brake the second filament lights up giving out more light. If you take a bulb out and look closely at it, you'll see the twin prongs inside! I believe they also have a staggered bayonet fitting so they'll only fit one way. So you would need to wire the LEDs to both contact points in the bulb housing - but it's all null & void anyway, because as with most incandescent bulbs these days I think you can buy them from eBay., you just need the model number 😉

EDIT: like these
http://pages.ebay.com/link/?nav=item.view&alt=web&id=360666053822

Not sure it t8ups does something that won't give an error, but no harm in PM'ing him.
 
Last edited:
Mine (3 door) doesn't use the twin filament bulbs but I know 5 door rear lights are different.
 
JJ.Foulds said:
So when i brake, the lights get brighter, now im guessing this is because the voltage is increased from 6v? to the normal 12v to make the brightness change.
Click to expand...

They work exactly as you describe, there is only one filament and it is controlled by Control Unit which limits the voltage when not braking and lets full voltage under braking. That's why you are able to set the brightness of the first one by VAGCOM (you set it as percentage of the full brightness. Mine is for example set to 22% when not braking). So yes, the LED strips would dim as normal filament bulbs.

But still I strongly advice you not to do what you intend as it will look rubbish and soldering few resistors into light housing may cause fire due to heat they produce. Not worth it in my opinion. And since the normal bulbs rely on reflections from parabolas your LED strips will not be as effective as normal bulbs, which may cost you in the end as some half blind/inattentive driver may run into you. Save up and buy OEM LED tail lights, they look terrific and you don't need to worry about them.
 
JJ.Foulds said:
ahh, see my electrical knowledge is limited but enough for some things.

Iv wired in LED's into my interior lights, and made my own DRL which come on when car starts. The interior ones work perfectly, might this mean the rear would also work, or do you think it wouldn't with what you said before?
Or should i just wire them up and do a test?
Click to expand...

Got any pics or write up of your home made DRL strips.. I'm intrigued!

Good on ya for having a crack at this sorta stuff, I wouldn't dare haha
 
atownsend91 said:
Ever thought of these?

Dectane Audi A3 8P LED Tail Lights Black Smoke | eBay
Click to expand...

Yes these are what gave me the initial idea as it looks like its just a LED strip inside, surely i can do the same!

fateman said:
But still I strongly advice you not to do what you intend as it will look rubbish and soldering few resistors into light housing may cause fire due to heat they produce. Not worth it in my opinion. And since the normal bulbs rely on reflections from parabolas your LED strips will not be as effective as normal bulbs, which may cost you in the end as some half blind/inattentive driver may run into you. Save up and buy OEM LED tail lights, they look terrific and you don't need to worry about them.
Click to expand...

Ahh thanks for the explaintion into the vcds % of bulb brightness, i was thinking to just use the LED for the normal lights then leave the brake light in housing still, but a resistor on a 21W bulb on a less than 6v wouldnt be alot of heat? but maybe wiring the LED's into the license plate wiring might be easier.

I realise the proper ones would look better, but im like super poor and cnt afford an audi nevermind extra's :p

RS3dan said:
Got any pics or write up of your home made DRL strips.. I'm intrigued!

Good on ya for having a crack at this sorta stuff, I wouldn't dare haha
Click to expand...

Yeah maybe i typed that wrong, iv not made the circuits myself, but bought really cheap ebay stuff and wiried it all in myself into the front footwell and under the rear seats, and in the boot.
 
fateman said:
They work exactly as you describe, there is only one filament and it is controlled by Control Unit which limits the voltage when not braking and lets full voltage under braking. .
Click to expand...
I seriously doubt this.

The effect is the same, but I am 99.9% certain you will find the voltage remains the same, but the duty cycle of the pulse width modulation (i.e. the time period the light is "on") is what allows the percentage brightness to be set. Reducing the voltage via some form of analogue means would require a big resistor or semiconductor(s) in the circuit and I'm fairly sure there aren't any in the Audi or other automotive circuits.

Actually LEDs don't respond well to using voltage to control brightness, so most LED circuits already use PWM - but the potential issue with this one would be that the lower %age used for "normal" tail light, the more likely you are to see visible flicker.

OP - I am not sure why you think you need a resistor at all if you are leaving the original bulb in place - if the LED strip is rated for a nominal 12v, the small additional current would probably still be within the canbus paramters. The only time you'd need a big resistor is to "fool" canbus into thinking there is still a relatively high current bulb present (if there isn't).
 
JohnE said:
I seriously doubt this.

The effect is the same, but I am 99.9% certain you will find the voltage remains the same, but the duty cycle of the pulse width modulation (i.e. the time period the light is "on") is what allows the percentage brightness to be set. Reducing the voltage via some form of analogue means would require a big resistor or semiconductor(s) in the circuit and I'm fairly sure there aren't any in the Audi or other automotive circuits.

Actually LEDs don't respond well to using voltage to control brightness, so most LED circuits already use PWM - but the potential issue with this one would be that the lower %age used for "normal" tail light, the more likely you are to see visible flicker.

OP - I am not sure why you think you need a resistor at all if you are leaving the original bulb in place - if the LED strip is rated for a nominal 12v, the small additional current would probably still be within the canbus paramters. The only time you'd need a big resistor is to "fool" canbus into thinking there is still a relatively high current bulb present (if there isn't).
Click to expand...

The reason id want a resistor was so i can remove the normal rear light completly, or most likely remove the parking rear light out the circuit.but while still leaving a bulb in place for the brake light.
 
Since we've established that the brake and parking light are on the same circuit and use the same bulb(s), not sure how you could leave just the brake light in place....
 
JohnE said:
Since we've established that the brake and parking light are on the same circuit and use the same bulb(s), not sure how you could leave just the brake light in place....
Click to expand...

Ahh yes... good point. not sure what i was thinking just then! aha.
 
So iv had a little play about with electrical blocks and some random led strips i found, unfortunately not red.

I can take a feed from the license plate lights then route it to each inner boot light first to see how it works.
Going to order some today then see if this actually works or looks good! (probably wont tho)

Sorry they are sideways, i couldnt get it to link correctly once id turned it.

So this is the electrical block in line with the number plate lights with just a blue led strip connected to test that it works correctly, which it does.


upload img

image
 
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