Driving common cathode LEDs from microcontroller

[Eric]

I'm a bit stumped on how to drive a bank of common cathode LEDs from a
microcontroller. Here is the problem.

The green and blue colors require 3.5v @ 40mA, and the red portion
requires 2v @ 40mA. I would like to drive 5 of each from the
microcontroller. For a normal bank of LEDs I would simple connect
them in series along with a resistor to the collector of a 3904
transistor. Since they are common cathode LEDs, I am guessing that I
would have to connect them in parallel between the transistor emitter
and ground. This would result in 15 (!) resistors. Is there a better
way to accomplish this task?

 

[Tim]

I'm a bit stumped on how to drive a bank of common cathode LEDs from a
microcontroller. Here is the problem.

The green and blue colors require 3.5v @ 40mA, and the red portion
requires 2v @ 40mA. I would like to drive 5 of each from the
microcontroller. For a normal bank of LEDs I would simple connect
them in series along with a resistor to the collector of a 3904
transistor. Since they are common cathode LEDs, I am guessing that I
would have to connect them in parallel between the transistor emitter
and ground. This would result in 15 (!) resistors. Is there a better
way to accomplish this task?

I do not believe you will find your idea practical. The output of most
microprocessors are a volt or so below the power supply. The NPN transistor
circuit you describe will cause at least another .6 volt drop. The green and
blue LEDs will probably not light under those conditions.

If the LEDs are requiring 40ma for all LEDs driven from one output, use a
2N3906 PNP or equivalent in the following circuit. If the requirement is
40ma each, use a TIP30 or equivalent.

To insure they light fully, use a PNP transistor with the emitter connected
to the microcontroller power supply, the collector to the LED bias
resistors, the base to center of a voltage divider (ladder) with one
resistor (about 1Kohm) connected to power supply, and the other (about
2Kohm) to the microcontroller output.

You may need to adjust the resistor values in the voltage divider so the
transistor is:
off when microcontroller output is high
and
on when microcontroller output is low.

Note the output/LED relationship will now be low-on and high-off. The
relationship can be reversed with a inverting buffer between the
microprocessor and the voltage divider resistor.

You can get fancy by using a higher voltage power supply (like 12vdc) and
connect the LEDs in series and use only one bias resistor. This will reduce
the current requirement to that of a single LED. This will require an
open-collector buffer or NPN transistor common-emitter circuit to activate
properly.

Hope that helps.

Tim

 

[Tim]

(snip)

You can get fancy by using a higher voltage power supply (like 12vdc) and
connect the LEDs in series and use only one bias resistor. This will reduce
the current requirement to that of a single LED. This will require an
open-collector buffer or NPN transistor common-emitter circuit to activate
properly.

Just a note. You must use individual LEDs for this series setup. A common
cathode array will not work in series as far as I know. And I don't know
much.

Tim