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Hooking up LED lights turn on like dome lights

Discussion in 'LEDs and Optoelectronics' started by Deski, Dec 9, 2016.

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  1. Deski

    Deski

    1
    0
    Dec 9, 2016
    Hey everyone,

    Electronics newbie here so I got some questions for you pros. I finished a project of replacing my headliner in my 2000 Cherokee XJ with a tapestry of a galaxy. Something different but looks real amazing! Even did the sun visors to match! After I got the headliner back in, I had this great idea to put some lights in the headliner to look like stars. What I'm looking to do is when I open the door, the lights (or stars) come on just like the dome lights. So my question is, how can I make this happen?

    I've posted on another electronic forum and the guys over there helped a little bit but I'm still having trouble figuring out how I can make this work. I was hoping I could get some more help from you guys to make this project happen. Let me get you guys up to speed on what I've tried.

    So these are the lights I want to hook up and put on my headliner:

    http://www.lightsforalloccasions.co...hts-timer-silver-wire-20-foot-cool-white.aspx

    It has 60 LED lights in the strain but I bought a smaller 5ft strain with 30 LEDs so I could use it as a guinea pig. The lights use a battery pack, using 3 AA batteries so it's only using 4.5 volts. Here's a few pics of the battery packs:

    The 5ft strain
    [​IMG]

    The 20ft strain
    [​IMG]

    The first thing I tried was just cutting the wires from the board and shoving the wires into the slots of the female 3 pin connector of the back dome light and got nothing. I then tried just touching the wires from the strip to the metal were the bulb goes and it would light up for a tiny second but go off. I definitely burned out the LEDs. Not realizing I have to take the 12v down to 4.5v. After posting on the other forum, someone recommend using some resistors to take the voltage down. So I purchased some 330 ohm and 390k ohm resistors to give it another go. I striped and wrapped the two wires together, soldered one 390K resistor to them and tried to touch the end of the resistor to the metal where the dome light bulb sits and no light. I noticed that the wires were hooked up marked with L+ and L- and the battery connections were mark V+ and V-. I didn't notice that before, so I'm guessing that L mean "Lights" and V means "Voltages"? I separated the wires again, soldered one 390K resistor on both wires, tired to touch the metal again at both ends of the bulb and still nothing. Could it mean the lights won't light up while touching the metal? Does it have to be directly on a hot wire? I asked if it would make things easier if I hooked up the lights with the board, and someone said that would make things easier to control, I just have to make sure to take the voltage down and he recommended to use a Buck converter. Again, with not much knowledge on electronics, I'm not sure which one to get to knock the 12v in down to 4.5v.

    I'm still not sure which wire I have to hook it up to make it work but I have found after some digging a few wiring diagrams that I'm not the best at reading so if that will help, I can post pics of those.

    I apologize for a lot of questions and information at first. I'm trying to get this project done before the end of the year. Any help would definitely be appreciated.
     
  2. Audioguru

    Audioguru

    2,983
    667
    Sep 24, 2016
    LEDs are pretty bright with 20mA each. For 30 of them in parallel you need 600mA but half (300mA) might be fine.
    Ohms Law calculates the resistor value to deliver 300mA from 13.8V in a car down to about 3.2V for white LEDs (the 4.5V battery probably drops to 3.2V when it is overloaded with 60 LEDs):
    (13.8V - 3.2V)/300mA=) 35.3 ohms so your 390k was 11 thousand times too high. 35 ohms is not a standard value but 33 ohms is close. The heating in the resistor is 300mA squared x 33 ohms= 3W so use a 5W resistor that will get pretty hot.

    The dimmer circuit in the truck might not work with these LEDs.
     
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