12 volt 8 Segment LED hookup to Microprocessor?

Regor

I have an 8 segment LED that requires 12 volts that I want to drive either from a microprocessor running at 3 or 5 volts.

I'm looking for advice on how to minimize the component count for doing this since the only way I can think of is having a Transistor for each output from the Microrocessor.

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Paul Baker

The simplest method is using a darlington array, however your LED must have a common anode configuration for darlington arrays to work.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Regor

That was my original thought process but the 8 digit LED requires a common anode.
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Regor

Meant to say common the 8 segment LED I have has a common cathode.

Paul Baker

I did some searching, pnp darlington arrays used to be made, but noone makes them anymore. You need to use discrete transistors, and there's an additional rub. You need to include a high side driver chip and transistors. This is because you are switching the high side, that high side is 12V, with a 5V microcontroller it's high side is 5V, thats 7V difference and enough to turn on any transistor. This means you have no way of turning off the transistors, this is where the high side driver comes in, it takes the 0/5V logic and translates it into a true on/off for the transistor.

The other option is to use a LED segment driver chip which can drive a common cathode, however none are made (that I could find) that work at 12V.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 4/30/2007 5:54:08 AM GMT

Regor

I really appreciate you taking the time to look into this. I've never used one but would an Optoisolator work for doing the voltage step up?

Mike Green

An optoisolator is an excellent idea.

Paul Baker

Yeah an optoisolator or a n-type transistor with a pull-up resistor:

    12V    12V
     ^      ^
     |      |
     \      |
     /      |
     \     /
     +---|< pnp
     |     \
     |      |
    /      LED
--|< npn    |
    \      ---
     |      -
    ---
     -

Actually come to think of it you can just use an optoisolator, since theres no galvanometric connection you can perform the control. Connect the cathode of the LED in the optoisolator to ground and connect the anode to your I/O pin (through a resistor first) you then can use the phototransistor to directly control the segment.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 4/30/2007 9:17:11 PM GMT

Regor

As a follow up I ordered a couple of Lite-On LTV-846 optoisolators.· They are working excellent.