Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Installing LEDs in footwells, will a diode work?

  1. Sep 24, 2012 #1
    I’m trying to install some LEDs in my footwells.

    I have a pink wire, which is my positive, and a gray wire, which is my negavtive. This is a negative switched circuit.

    So, is it ok to hook it up like this?

    1 Positive of LEDs to pink, negative to gray.

    2 Then, use a diode on the gray wire, with the stripped end pointing away from the leds back to the gray. Now, on the non stripped side of the diode, add a wire which connects to a switch. Then, connect the other side of the switch to the ground on my cigarette lighter?

    The first part works I know. But can I just use a diode? Or do I need a relay? I know the second step functions I just want to know is it safe?

    Also, if the second isn’t safe, would it be safe to ground it to the car instead of into the cigarette lighter?

    Thanks!

    Oh- finally, if that does work, does the ground of the cigarrette lighter function as a ground 24-7? Or only when the car is on?
     
  2. jcsd
  3. Sep 24, 2012 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    the ground of most cig. lighters is physically connected to the chassis ( metalwork of the car. It would be very rare for cig lighters to come through plastic as there would be a hi chance of the heat melting the plastic surround.
    On rare occassions I have seen a short wire from the outer casing ( negative) of the cig lighter to the car chassis

    Hope you used resistors in series with the LED's else you mite find that have a short life
    ~ 1k Ohm (1000 Ohms) per LED for 12V.
    Maybe you are using commercial LED strips that already have resistors built in ?


    Dave
     
  4. Sep 24, 2012 #3
    Yes I'm using some commercial LED strips I got on sale.

    Does my setup sound ok with using the diode like that? Or do I need to worry about any sort of problem between the dome light and cigarette lighter?
     
  5. Sep 24, 2012 #4

    davenn

    User Avatar
    Science Advisor
    Gold Member

    OK just trying to figure out the earlier part of your post
    and then your latest comment about the dome light

    some questions for you to answer.....

    1) when do you want this LED strip to light up
    all the time?, only at nite ? only when the car door opens ?

    2) that pink wire, presumable a wire from the car wiring loom ?
    do you know how its powered/ comes from ?
    Does it have power on it ALL the time or only when the car ign is in ACC or only when the ign is full on ( car running)

    3) the grey wire where does it go to/come from? is it at 0V/chassis GND ?

    4) Not sure what the dome light has to do with it ?? you didnt mention that initially

    Dave
     
    Last edited: Sep 24, 2012
  6. Sep 24, 2012 #5
    I'll make this easier and answer below (Also I appreciate the help!):

    1) when do you want this LED strip to light up
    all the time?, only at nite ? only when the car door opens ?

    When the doors open, and then when I have the car on, with a switch so I can turn it off.

    2) that pink wire, presumable a wire from the car wiring loom ?
    do you know how its powered/ comes from ?
    Does it have power on it ALL the time or only when the car ign is in ACC or only when the ign is full on ( car running)

    The pink wire has power all the time, it goes to the dome light, sorry forgot to mention.

    3) the grey wire where does it go to/come from? is it at 0V/chassis GND ?

    Grey wire goes to the dome light. It is 12v until the door is opened, at which time it becomes grounded.

    4) Not sure what the dome light has to do with it ?? you didnt mention that initially

    Sorry, didn't make that clear, the pink and gray wires go up to the dome light. So if we were to ground the gray wire, it would turn on the dome light, so I need the diode.

    Let me know if there is anything else I need to explain better.
     
  7. Sep 24, 2012 #6

    vk6kro

    User Avatar
    Science Advisor

    Wouldn't you just want the LED strip to come on at the same time as the dome light? That is when you open the door or if someone turns the light on with the switch?

    If so, then you need to connect the LEDs directly across the dome light.

    The circuit for the dome light is just 12 volts positive, through a fuse, through the dome light and then through a door switch to ground. This switch has another on-off switch in parallel with it so that you can use the light for map reading etc.

    My car also has a timed switch so the light stays on for a minute or so after you leave the car, presumably so you can find your way to the front door of your house.
     
  8. Sep 24, 2012 #7
    I do want it to function like that, but I do also want it to be able to be on while I'm driving. They are very dim so they're not at all a distraction, so I would like to be able to do that.

    So knowing that, how does all that look?
     
    Last edited: Sep 24, 2012
  9. Sep 25, 2012 #8

    vk6kro

    User Avatar
    Science Advisor

    All that would be possible. You just have a PNP transistor that turns on when the door switch closes. You do that by connecting the base resistor of the transistor to the non-grounded side of the switch, via a diode.
    When the door opens, the switch closes so base current flows and the transistor turns on, turning on the LEDs.

    Then, you have another switch and another diode and base resistor which you can operate when you want the LEDs on when the door is closed.

    HOWEVER, unless you have some experience with this sort of circuit, you would be better off just getting a cigarette lighter plug and putting a switch in series with the LEDs so you can turn them on whenever you want to.
     
  10. Oct 18, 2012 #9
    why not like this?
     

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Installing LEDs in footwells, will a diode work?
  1. How Diodes work? (Replies: 5)

  2. LED vs plain diodes (Replies: 5)

Loading...