led woes

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=102514
Printed Date: May 14, 2025 at 3:05 PM

Topic: led woes

Posted By: iblis
Subject: led woes
Date Posted: February 24, 2008 at 3:33 PM

Hello all. Hoping someone can tell me what I'm doing wrong here.

I've been experimenting with trying to put an LED (built in resistor for 12V) in place to indicate whether the fog lights are active.

The two scenarios are as below:
posted_image

In the series scenario, the LED lights, but the fogs do not come on.

In the parallel scenario, the fogs come on, but the LED does not.

Neither of these are quite what I had hoped for. posted_image

I'm sure I've done something silly here, but my brain has turned to mush after ages of looking at this. Any help very much appreciated! Would be nice to be able to reassemble the interior of the car posted_image

Thanks

Replies:

Posted By: i am an idiot
Date Posted: February 24, 2008 at 5:49 PM

Connect the positive lead of the LED to the same place you connect the fog light positive wire. Ground the LED to ground.

Posted By: iblis
Date Posted: February 24, 2008 at 6:02 PM

Thanks for the reply. Do you mean to connect it in parallel with the fog lamp?

posted_image

Posted By: KPierson
Date Posted: February 24, 2008 at 7:06 PM

If you want to stay on the trigger side wire the LED in parallel with the relay coil (the + of the LED to 12vdc and the - of the LED to the (-) switch).

Or, you could do the above and you would be on the switch side of the relay, which, at the end of the day, is the important part to monitor.

-------------
Kevin Pierson

Posted By: i am an idiot
Date Posted: February 24, 2008 at 7:09 PM

As Kevin mentioned either way will give you what you need.

Posted By: iblis
Date Posted: February 25, 2008 at 1:57 AM

That's great, thanks guys! Your help is much appreciated.

Any ideas on what went wrong the first time? Be great to learn something from this experience.

Posted By: iblis
Date Posted: February 25, 2008 at 4:14 AM

Would I be right in assuming that in the series situation from my original post the resistance was too high to energise the coil, but there was enough to light the LED? And in the parallel case, the current was finding earth so never going through the LED?

Thanks again!

Posted By: KPierson
Date Posted: February 25, 2008 at 5:30 AM

Since you have a 12vdc LED it has a current limiting resistor built in. When you put that in series with the coil of the relay it prevents enough current from flowing through the relay coil to energize the relay. Had you gone with a 'normal' LED it may have worked.

The parallel setup has no way for current to flow, as you are connecting both leads to the same signal. You need a difference of potential to turn the LED on.

So your assumptions are correct!

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Kevin Pierson

Posted By: iblis
Date Posted: February 25, 2008 at 5:42 AM

Thanks Kevin!

Really appreciate you guys taking the time to reply. This could have taken quite some time for me to resolve on my own!

Ah well, at least I'm learning posted_image

Cheers