Hey y'all, probably in the wrong place to ask this question, but here goes...

I'm working on a little project, using an AD DSP controller..
trying to use a single GPIO to control a two state led..
and of course, this should apply to any microcontroller using 3.3V GPIO

I found this circuit a while back in some online search, and did a screenshot of the schematic, and recently built up the circuit, and it almost works...

And of course, now I can't find the original article where I found the circuit. and only difference is 12V vs 24V powering the circuit.

Seems pretty straight forward, using a 3-lead/3-color LED (common cathode), power one side green, power the other side red, (and power to both yellow)...

The circuit consists of 2 pnp transistors driving the LED's, one biased on and the other off, and an npn transistor driven from a GPIO that makes the 2 pnp's change state, so should change from red to green..

But somehow there is enough leakage from the "on" transistor that the "off" state is yellow, while the "on" state makes green. ok, cool that it changes state, but the visual difference between green and yellow is sometimes hard to distinguish in bright light, would rather have a distinct green/red..

Any suggestions??

JohnR/stagekraft

 

Any suggestions??

Measure the voltages at the collector of the transistors and the base of the transistor that's supposed to be off.

It might also be helpful to know the values of R1 and R2.

 

The turn-on path for Q3 is through D1 via R5 and R1. This is probably where the problem lies.

If your MCU can drive a LED directly from the IO pin, then this would be worth a try:

 

Try this:

 

Try this:

Won't that still turn on D2 slightly from Q1's base current?

Below is the LTspice sim of a circuit that prevents a sneak current path through the off LED:
It drives one of the LEDs with an NPN emitter-follower.

 

Try this....

Doesn't that still have parasitic turn on through the LEDs??

 

This approach has worked for me many times.

Depending on the GPIO pin capabilities, the center schematic might work for you without the inverter driver. Connect the D3 cathode directly to the GPIO pin.

The lower group of schematics is the same idea re-worked for a common-cathode LED.

ak