Need help with project using LED lights.

Mark42

Fleet Admiral
Joined
Oct 8, 2003
Messages
9,334
The dark carpet in my car makes it imposable to see anything on the floor at night. So I want to add some lighting under the dash and rear seat floor area when the dash lights are on. Want to use LED lights so the additional load on the circuit is minimal.

I found these LED lights at Radio Shack, and I bought two to play with. They are 10 mm super bright LED's.

pRS1C-2160473w345.jpg


The specs on the bag are:

Absolute maximum ratings:

Forward current(if) 40mA max.
Forward Voltage(Vf) 5VDC max
Reverse voltage 6VDC
Power dissipation: 100mW

Electro-optical characteristics:

Forward voltage: 2.4V +/- 03V
Peak Emission wavelength 660nm
Luminous intensity 5000mcd (typ at 20mA)

The problem is how to make this 2.4V LED run on 12V?

In the past, I had added some LED's indicator lights, and was told to use a resistor to cut down the voltage to the LED (this from a guy working at radio shack years ago). So I connected the new LED using some of the same resistors and it works, but I'm not sure how long the LED will last with this setup.

I tested the resistors, and at 2K, they show 0.501 ohms on the multimeter.

Is this a safe way to connect the LED's? Will they last or burn out soon?

Thanks for any suggestions!

Mark
 

madgadget

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Sep 28, 2009
Messages
281
Re: Need help with project using LED lights.

You need to put your meter in current mode (mA) and start with a resister about 800R (ohms) and work backwards (lowering the resistance) until you have somewhere in the reigon of 30-40mA from a 12.5-13v supply.

It would be possible to calculate an approximate resistance of say the following requirements:

V supply = 14v (engine running)
Led 5v
Current 40mA

So 14-5=9v

resistance=volts/current

9/0.040=225 ohms.

In reality I would be amazed if using a 225 ohm resistor provided worked. I suspect you will need around 300-380 ohms. Give it ago and see how you get on.
 

i386

Captain
Joined
Aug 24, 2004
Messages
3,548
Re: Need help with project using LED lights.

Here's one of many LED resistor calcultors available online.

http://ledz.com/?p=zz.led.resistor.calculator

If I did it right, you need a 270R .5W (or greater) resistor. That's just for a single LED though. Madgadget's method will get you there too.


I know a few sources for little bezels to hold the LEDs if you're interested. Not sure if Radio Shack has them. They should carry your resistors though.

Cool project.:)
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: Need help with project using LED lights.

These guys are figuring on the absolute max specifications, which cannot all be reached at the same time. For instance, 5V @ .040 amps = .200 watts, not the .100 maximum spec.

Design point should be 2.4v at .020 amps.

14.4 - 2.4 = 12v resistance drop.

12v / .020 amps = 600 ohms.
12v * .020 amps = .24 watts.

diode dissipation 2.4v * .020 amps = .048 watts. About 1/2 absolute maximum rating.

Nearest reasonable dropping resistor with margin would be 620 ohms at 1/2 watt. 560 ohms, wouldn't exceed max specs and would give a brighter light.

Likely the resistor you tried was 510 ohms 5%, (green, brown, brown, gold). that's about the limit, but should be OK.

hope it helps
john
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Need help with project using LED lights.

Mark take a look at these. I have them in my boat for night lighting and they are great. Waterproof, good looking, easy to mount and connects directly to 12V. THey come in white, blue, red and amber although this supplier only carries the two colors.

No messing around with resistors and such. Plug and play :D

http://www.westmarine.com/webapp/wc...rue&storeNum=5002&subdeptNum=9&classNum=13045
 

ThumbPkr

Petty Officer 1st Class
Joined
Aug 17, 2007
Messages
371
Re: Need help with project using LED lights.

Here's one of many LED resistor calcultors available online.

http://ledz.com/?p=zz.led.resistor.calculator

If I did it right, you need a 270R .5W (or greater) resistor. That's just for a single LED though. Madgadget's method will get you there too.


I know a few sources for little bezels to hold the LEDs if you're interested. Not sure if Radio Shack has them. They should carry your resistors though.

Cool project.:)

Those little bezels can really add some class to your project.I just built a shutter speed tester for film cameras and just happened to have a couple of those on hand from the old days and it made the job look so much better.I don't know if the "Shack" still carries them or not.LOL.Ron G
 

Mark42

Fleet Admiral
Joined
Oct 8, 2003
Messages
9,334
Re: Need help with project using LED lights.

Thanks for all the input! Should be able to get the right resistors now.

The lights from West Marine will be just too bright for night time driving. Need something a bit dimmer.

Thanks again for taking the time to help me out.

Mark
 
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