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, 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 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.