Please Help Me with Vehicle Glove Box LEDs

Discussion in 'The Projects Forum' started by JohnnyGlock, Jun 7, 2009.

  1. JohnnyGlock

    Thread Starter New Member

    May 23, 2009
    3
    0
    I am very new to electronics as a hobby and was wondering if someone out there can possibly point me in the right direction.

    I wired in some custom LEDs replacing my car's stock incandescent bulb. It looks awesome, but then I found out that the car's circuitry is such that the glovebox light is always on (with the headlights on)!!

    I would like to wire in some kind of switch so that the light is only ON when the box is OPENED and OFF when the box is closed.

    Trying to figure this out, I picked up some "momentary tact" switches (http://www.radioshack.com/product/index.jsp?productId=3020763), and a "detect switch" (http://www.radioshack.com/product/index.jsp?productId=3190382) from radio shack. I like the detect switch because it's small and I think would be best for this application, but it's rated for only 5VDC/1mAh. So how can I use it in my car?

    I also picked up some transistors, as I heard they can be used as switches, and I got some photocells to experiment with them. But as it turns out the photocells resist TOO much (they need too much light to lower the resistance enough to light up the LEDs). And since these lights will most likely only be on at night, I'm guessing that's not a good idea here?

    Finally, I saw a magnetic switch alarm at the dollar store (one of those types where you fasten onto windows and doors), so i picked one up, thinking that I can use the magnetic switching mechanism detached from the speaker/alarm. But when I tried that, I realized the two wires going to the speakers, alternate SOMETHING (voltage? current? idunno), I guess because the speaker membrane has to oscillate back and forth to make that shrieking alarm sound, so when I tested with an LED it would just blink really rapidly and I think it might have killed it completely.

    What would be my best option here? Thanks again for your help in advance.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    If you are going to use LEDs there really isn't any reason to turn them off, they are much more efficient than incandescent light bulbs.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    Focus on the LED section.

    If you just have to use a switch I'd using a momentary contact Normally Open type.
     
  3. JohnnyGlock

    Thread Starter New Member

    May 23, 2009
    3
    0
    Thanks for your quick reply. And I have read that LED section, which helped me when I first started reading up on LEDs.

    But the reason I want them off is not due to worry about power consumption or anything like that, it's just the lights are bright and shine through the glove box's small openings.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Since the light only lights with the headlights on this means you won't be sucking current just sitting there. Like I said a momentary contact switch (ie, push button) that is Normally Closed would do it, as it makes contact when it is not pressed. I misstated myself on the NO contacts before, for which I apologize.

    Oddly enough, there is a way to use a Normally Open switch for this function. If you want to use a reed switch it closes in the presence of a magnet, which is the same as a NC switch. You can use this switch to short the LEDs, add a resistor to drop the voltage below the voltage turn on of the LED you can keep the current down too. The LEDs shown below are assumed to be white.

    [​IMG]

    I'm not absolutely sure this will work, but I think it will. I'll run some simple experiments and get back with you.

    The reason I feel the 270Ω is needed is wattage. With the LEDs on the 300Ω resistor will disippate 1/8W, if it is by itself it will disappate over ½W, which is too much heat IMO.
     
    Last edited: Jun 8, 2009
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, it did not work, but when I dropped the power supply voltage from 13.6V to 10V it did, which suggests a lower resistance instead of 270Ω will do the trick.

    If you decide to go with this scheme use a ½W resistor, and you can take a pot and find the crossover where the lights dim out considerably, then measure the pot.

    You could also use with a transistor to do the switching, I was trying to keep it simple. The transistors will be a lot more efficient though.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, I went through the following logic. If the light turned on at 10V, then the resistors dropped 4.7. I treated this as a threshold voltage. So to drop 4.7V at 13.6V I needed a 156Ω resistor. The logic is shaky, but I tried it, and it seemed to work. With the power supply at 13.6V and the 270Ω replaced with 180Ω the LEDs stayed lite, with 160Ω they were dim, but lite, and at 150Ω they were mostly dark (a small glow). So you can turn the LEDs off by using a N.C. switch and two resistors, 150Ω and 300Ω.

    Sometime I'll put the LED/resistor bargraph up and show how that works.
     
Loading...