Powering an array of leds from one single MC output line

[weyes]

I am building the master caution button for the 767LD. The button is composed of 2 arrays of 4 leds each. One array for the red and the other for the yellow.

My idea is to connect both the array to one ouput line of the MasterCard. It basically means that one single output line of the MC should be pwoered enough to turn on 4 leds.
You can get the schema of what I have built by using this web site (http://led.linear1.org/led.wiz), given that one array is composed of 4 leds, 20mm 3V each.

By talking with a friend of mine, he said that it could be that the MasterCard can't do that but I did not found any technical specification of the lines of the MC apart that each of them is a 5V line.

[fordgt40]

Alessandro

OC advise that the maximum current that can be drawn on a mastercard output is 20ma. I suggest that you try 4 leds with a series resistor to limit the drain to 20ma and see how bright they are. If this is unsatisfactory, then it is possible to use a darlington pair of transistors fed with a separate supply to provide sufficient current.

Regards

David

[weyes]

I see... so I am completely out of the specification given that each LED is 20ma.

I was thinkning to use the array together with some kind of relais then because I can have a 5V power line outside of the MC.

[fordgt40]

Alessandro

Try it anyway and see how bright it is. I use two leds for the annunciators with a total current drain of 20ma on one output line and it is ok. I assume that you are short of output lines and could not drive 2 leds from each of two outputs lines.

Regards

David

[weyes]

Well, I have enough output lines and in the worst case I could wire the 8 leds with 8 outputs.

The problem is that I am not expert in electronic and I did not think at all about the ampers: I focused on the 5V only :(
Now the cabling is done and I should redo everything from scratch. This is why I am thinking of a couple of relais

I have tested the arrays with a LED tester having an output of 20ma and they work even if the light is less then I expected. After that I have connected the array to the PS and it is much better.
I want to check if the solution with two relais is ok because I would like to have the MC very bright.

[weyes]

I could probably use an operational amplifier (http://en.wikipedia.org/wiki/Operational_amplifier) given that I can easily have a 5V and a 12V lines from the power supply. This might allow me to reuse the array they are already wired.

[fordgt40]

Alessandro

Yes, that is what I was suggesting with a darlington pair. I think you need to get an IC no ULN2004. This is an array of 7 transistor pairs on a chip that allows each output a current of 500ma. Basically, you connect your OC output, probably through a diode, to the IC ULN2004 input. The ULN output will give a output voltage of between 5-12v dependent on what power source you give the chip.

Regards

David

[weyes]

Thank you again David.
I didn't get your suggestion at the beginning problably because of my very low level of knowledge of electronic.

I will go ahead with the work and let you (and the others) know.

[fordgt40]

Alessandro

Good luck and let us know how you get on. I will probably have to go the same route to provide enough power to light up the bulbs in the pedestal fire handles

Regards

David

[Perik]

Alessandro

Don't forget to look at Mike's site for ideas - like this one:

http://www.mikesflightdeck.com/annunciator_lights_1.htm

or equivalent. One transistor and a couple of resistors should do the job.
The advantage is that these few components can be placed inside the
switch - just supply the signal, 12V and GND.

[Jackpilot]

Alessandro

Good luck and let us know how you get on. I will probably have to go the same route to provide enough power to light up the bulbs in the pedestal fire handles

Regards

David

Super bright leds work well too in the FH and can even blink (with a hidden blinking led in serie). Much lower load altogether.

[fordgt40]

Jack

Now thats a great idea - I did not think about putting a small blinking led in series. That solution would be handy elsewhere as it is difficult to get high output blinking leds

Regards

David

[fweinrebe]

Yes, that is what I was suggesting with a darlington pair. I think you need to get an IC no ULN2004. This is an array of 7 transistor pairs on a chip that allows each output a current of 500ma. Basically, you connect your OC output, probably through a diode, to the IC ULN2004 input. The ULN output will give a output voltage of between 5-12v dependent on what power source you give the chip.

Have a look at this card. It is actually for a PIC development board, but it could be used for what you are referring to. It uses a ULN2804 and has typical power loads totalling over 260 W (350 mA x 8). http://www.mikroe.com/en/tools/mikrodrive/.

[pdpo]

Just for your info,

mastercard can drive a LED with a resistor in series to limit the current through the LED.
20mA is a rather high value for a normal LED but for the superbright LEDs the current can
be higher.
Measurements on an output have revealed also an internal resistor on each output. This
results in the fact that on a 5 Volt output 2 leds in series can be connected without the
use of a resistor and the current will be limited enough by the internal resistance.
When you need to drive more then two leds then you could use two optocouplers connected
with their leds in series. This way the MC output will light the two leds in the optocouplers which can drive then a 12 Volt line with 4 leds in series with a small current limiting resistor.
this way you can drive 8 leds from one MC output.

Greetz Peter

[weyes]

Thank you everybody for your help.

In the mean time I have discovered that the button is splitted horizontally. The yellow line (warning) is at the bottom; the red line (caution) is in the upper side.
This triggered a whole refactoring of the button. In particular I can put in each of the 2 lines no more then 2 LEDs because in fact I do not have enough room for more then 2 LEDs.

Initially I try feeding the 2 LEDs with the solution prosed by Peter (pdpo).

I need some time now for the connection of the cables but I'll keep you up to date.

[weyes]

Just an update...

The button is ready. The two LEDs worked as Peter suggested and the final result is even better then I thought... I am quite surprised of the final result, much better then I could thought. I have taken some pictures and when I have time I will post a kind of tutorial in my blog and add here the link.

However, the problem of lighting several LEDs at once is waiting me for the gear panel. But that's another history...

[weyes]

I have put some photos describing the work I did in my blog. If you're curios you can red the last post of the blog (http://weyes-homecockpit.blogspot.com/2009/03/master-caution-button-complete.html).

The cost of the button is around 10-12 euro. I guess this same solution could be used for korry switches too.