Light Resistant LED Question

[Siegfried Meier]

Hey guys!

Not an audio question, but an LED question that's baffling me and I was hoping someone could help here.  I had a problem with the 3rd brake light on my VW Golf - the one inside the hatch.  Upon opening, it's a long bank of 4 pin odd looking LED's.  The issue is that it all works if all the LED's are getting enough light, but the second I put the red cover back on, and it's not getting light, only 6 of the 18 LED's light up.  Initially I thought it was wiring, but I confirmed that it's definitely light that's doing it.

Why does a brake light need to be light sensitive to begin with?  Shouldn't it just be on…or off?  Or is the light sensitive LED's useful for making it brighter during the day?  Am I going crazy here?

When they're on and have power, they're all on.  I wish I could just make them stay that way instead of being light senstive…

Any help much appreciated!
Thanks,
Sig

 

[Gus]

Are the LEDs SMDs and on a PCB?
If so maybe the LED connections are opening due to some kind of movement from  forces when the cover is installed.

 

[JohnRoberts]

Hey guys!

Not an audio question, but an LED question that's baffling me and I was hoping someone could help here.  I had a problem with the 3rd brake light on my VW Golf - the one inside the hatch.  Upon opening, it's a long bank of 4 pin odd looking LED's.  The issue is that it all works if all the LED's are getting enough light, but the second I put the red cover back on, and it's not getting light, only 6 of the 18 LED's light up.  Initially I thought it was wiring, but I confirmed that it's definitely light that's doing it.

Unlikeley... 

Why does a brake light need to be light sensitive to begin with?  Shouldn't it just be on…or off?  Or is the light sensitive LED's useful for making it brighter during the day?  Am I going crazy here?

Again unlikely... If they did that they wouldn't do it locally, but globally with a single light sensor.

When they're on and have power, they're all on.  I wish I could just make them stay that way instead of being light senstive…

Any help much appreciated!
Thanks,
Sig

+1 to what Gus suggested. Check for a mechanical difference between open and closed that explains difference. Like a cracked PCB, dry solder joint, loose wiring connector, etc.

JR

PS: Back in the '70s we received some experimental BBD devices from Phillips that were in open top IC cans. We discovered by accident that the (fluorescent) bench lighting corrupted the electrical signal passing through the BBD.

 

[Andy Peters]

PS: Back in the '70s we received some experimental BBD devices from Phillips that were in open top IC cans. We discovered by accident that the (fluorescent) bench lighting corrupted the electrical signal passing through the BBD.

And that is how the CCD image sensor was invented.

-a

 

[Siegfried Meier]

Well, I have had the open PCB sitting here in my shop and powered with a wallwart…if it's in the dark, only 3 light up.  If I have my bench light shining on it, they all light up…so this is definitely an odd phenomenon…

It's toast either way, I'll probably just have to find a new one at the wreckers.

 

[JohnRoberts]

Well, I have had the open PCB sitting here in my shop and powered with a wallwart…if it's in the dark, only 3 light up.  If I have my bench light shining on it, they all light up…so this is definitely an odd phenomenon…

There are optical switches that are turned on by light. It is extremely unlikely for this to happen by accident. Any such accidental photo effect should be small and unlikely to drive a relatively high current lamp.

Is it possible it is supposed to work this way to light up brighter when it sees a headlight shining on it?

It's toast either way, I'll probably just have to find a new one at the wreckers.

Another obscure possibility is you are seeing reflected light coming back?

It's always something.

JR

 

[benb]

There are optical switches that are turned on by light. It is extremely unlikely for this to happen by accident. Any such accidental photo effect should be small and unlikely to drive a relatively high current lamp.

Is it possible it is supposed to work this way to light up brighter when it sees a headlight shining on it?

Boy, does this ever make sense!

Car designers of years and decades earlier, had they ever thought of something like this, would only have dreamed of adding a circuit to make a brake or running light brighter in the presence of more ambient light. Now that these lights use  LEDs on a (almost certainly) SMT PCB, adding a phototransistor a couple resistors and a transistor switch for an extra LED is cheap and easy.

I was about to suggest trying out the other brake light to see if it does the same thing, but rereading the OP,  this is the "third" brake light in the middle, the one in most cars shining out at the bottom of the rear window. It's possible that the other two brake lights work like this, so you might try them. You might try reading the car's owner's manual to see if it says anything about this phenomenon.

Or you can find another Golf at night, ask the driver to press the brake light, count the lit LEDs, then shine a flashlight in it and see if an extra LED lights, just like the one on your bench.