Best way to backlight

[BuildABoeing]

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?

[mickc]

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 (http://www.ebay.com.au/itm/120846695337?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_4108wt_1398)

[jonesthesoftware]

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 (http://www.ebay.co.uk/itm/LED-Light-Dimmer-Brightness-Adjustable-Control-12V-8A-/290595258515?pt=UK_Light_Fittings&hash=item43a8d3c093)


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

[fordgt40]

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

Flexible LED strips 24 way (http://www.rapidonline.com/Electronic-Components/Flexible-LED-strips-24-way-81567)

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

[tiburon]

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.

[fordgt40]

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

[jonesthesoftware]

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

[fordgt40]

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

[jonesthesoftware]

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

[davemuir]

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

[BuildABoeing]

Guys! Guys! Guys! :-)
I am getting all confused! :shock:

First of all I - of course - do not like what you are saying. :-) I have backlit my whole pedestal on the solution of "2 leds connected to 5V", and now you all say "Go 12V". :-)

Is it just a question of providing 3V per LED in the series?

12v you say, and some of those dimmers from eBay, that is it? And then 4 LEDs in series?
Like this?
[+12v ] ----> [Dimmer] ----> +LED ----> +LED ----> +LED ----> +LED ----> [GND]

That should give 3V per LED, or do I need resistors?

I got all confused along the track when you started talking about that dimmer. :-)

David, thanks - exactly that colour!

[davemuir]

Hi Peter,
I have found the easiest way of doing the backlighting
Is to use led strips it comes on a reel and you can cut every 4 LEDs in length .
it has all the resisters built in and when you cut the strip you can join them in series to any size and shape of panel behind the mip.
i use Perspex clear panels behind the mip ,the led strip is also sticky back
easy and quick -
Best regards
dave muir

[fordgt40]

Peter

Sorry you are having problems. Basically, there are two options with leds for backlighting

Use 12 volt strip leds, no resistor needed and the EBAY dimmer will work fine

Use a 5 volt supply with two leds in series or parallel PLUS a current limiting resistor. The EBAY dimmer may or may not work, as they are designed for 12 volts, but no harm trying. You can get the right resistor value here

LED Resistor Calculator (http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator)

EDIT You could of course also use a 12 volt supply and power more leds in a "set", with the correct current limiting resistor (as outlined by Geoff), in which case the EBAY dimmer would work ok

David

[jonesthesoftware]

Here's some examples of the 12V route
7446


regards
geoff

[BuildABoeing]

Hi again guys!

Thanks a lot for clarifying - I really appreciate it!

It is not that I have run in to problems (Except understanding the technical aspect of the dimmer :-) ) I just was not quite satisfied with my pedestal-backlightning (The 5v 2LED super-bright solution) so I thought why not get some advise from the best (= You guys!) :-)

I'll get some of those LED strips.
It also seems easier to work with (Guess it just can be glued on a backplate) rather than making separate holes for 10-15 LEDs per panel.

Thanks guys!