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  1. #1
    75+ Posting Member BuildABoeing's Avatar
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    Best way to backlight

    What is the best way to back light the B737 panels?

    So far I have connected two white LEDs to 5V. Like this:

    [ + 5v ] ------> +LED- ------> +LED- -------> [GND]
    It works fine.
    But adjusting the brightness with a rotary pot is almost impossible. It is either all or nothing.

    How have you guys done?

    I would like to be able to dim the brightness.

    Further more the super bright White LEDs give a very (aaahr welll...) white light, not the warm yellow that can be seen on real panels. So I might have to use other coulor LEDs. Any advice on which LEDs to use?
    Best wishes, Peter
    Building a B737NG - Blog: BuildABoing

  2. #2
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    Re: Best way to backlight

    For dimming LEDs you need a dimmer circuit.

    If you just use a pot with an LED, it will dim very slightly until the supply is under the LED's forward voltage rating, then it will just go out.

    I use cheap LED dimmers off Ebay. They are very simple, just feed them 12VDC. You can take them out of the case easily, just a very small board, an you can relocate the pot in them to anywhere you like in your cockpit.

    I picked up 5 of them for $1 each !
    eBay Australia: Buy new & used fashion, electronics & home d

  3. #3
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    Re: Best way to backlight

    Hi
    first you MUST put resistors in series with leds. Leds are a short circuit once the voltage across them is greater than 0.7 volts.
    You can calculate what value resistor to use like this
    R= Vs-Vf/I
    where Vf= forward voltage of diode from datasheet generally 2-3.2 volts
    Vs = voltage of supply used to drive leds
    I= desired current generally in the order of 20mAmps
    e.g for 20 mA using 5 volt supply and 3.2 Vf diode =
    5-3.2 = 1.8
    1.8/0.02=90 so use 100 ohm resistor

    if you are driving your leds from an output card you can normally only use 1 resistor and 1 led per output as only 5 volts is available and if you deduct the 3.2 volts for the first led there's not enough volts left for a second led in series with it. There are some exceptions for example I use yellow leds with a Vf of 1.6 volts so I can get 2 in series from a 5 volt digital output as
    5-(1.6+1.6) =1.8
    1.8/0.02= 90 ohms

    if you the dome head leds file the top flat to diffuse the light a little better and use a PWM dimmer( pulse width modulation) which you can buy very cheaply on ebay like this one..

    LED Light Dimmer Brightness Adjustable Control 12V 8A | eBay


    I believe most cockpit builders myself included, use 12 volt for backlights as you can use multiple leds in series. Again calculate the resistor needed.
    hope this helps
    regards
    geoff

  4. #4
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    Re: Best way to backlight

    For general panel backlighting it is best to use 12volt led strips as per these examples

    Flexible LED strips 24 way

    You will need leds with a light spread of at least 120 degrees

    See this link for how I backlit my overhead

    http://www.737ng.co.uk/djb/Overhead%20Article.pdf

    If you are using a typical I/O card output then you can use two leds wired in parallel from a single output of 5 volts, there are plenty of led resistor calculators on the web. Also, if you are using leds for annunciators then make sure you get flat top leds, ordinary leds are generally very poor with a light spread of only 15/20 degrees giving you pin point light effect

    Regards

    David

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  6. #5
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    Re: Best way to backlight

    You can buy 5 mtr warm white led strips on ebay for about 18 euros. Do a search for the 5050 type leds, those are the high intensity ones.

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    Re: Best way to backlight

    Quote Originally Posted by mickc View Post
    For dimming LEDs you need a dimmer circuit.

    If you just use a pot with an LED, it will dim very slightly until the supply is under the LED's forward voltage rating, then it will just go out.

    If you follow this advice with a single led and 12 volt supply (as suggested) then it will go "phut"

    These EBAY dimmers will just blow up a single led on a 12 volt supply. They are for led strips or lots of leds in series where you have already done the calcs

  8. #7
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    Re: Best way to backlight

    Hi forgt40
    no disrespect but the dimmer works fine even for 1 led as I use 4 of these dimmers, but you MUST still use the series resistors. They work by PWM which pulses the output by changing the frequency of the ON and OFF states but it does not reduce the voltage which stays at approx 12 V

    regards
    geoff

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    Re: Best way to backlight

    Geoff

    No disrespect taken I fully understand how the dimmers work

    The point was that the poster did not mention using current limiting resistors!! Indeed, he said "If you just use a pot with an LED"

    Regards

    David

  10. #9
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    Re: Best way to backlight

    Hi David
    yes you're right, the most important fact of using leds is the current limiting resistor. Most jobs and designs I've worked on have used leds with 20mA current and most data sheets for leds show this as a typical value so I wish manufacturers would "build in" the CL resistor. This seems feasible give how small SMD resistors are, and would make my life easier with all the Korrys and annunciators and backlighting leds I've used

    kind regards
    geoff

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    Re: Best way to backlight

    Hi peter
    check out my youtube Chanel at 1davemuir
    i used warm white led strips with a 6 volt power pack easy
    regards
    davemuir

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