Semaphore using one microcontroller pin

Question

I'm trying to make a semaphore using to LEDs green and red, controlled by one arduino pin. Idea is when pin is low red LED is on and green LED is off, when pin is up green LED is on and red LED is off.

So I've made the circuit using two transistors and in simulation software everything seems to be ok, but on practice red LED (right one on the picture) is still shining dimly when the green one (left one) is on.

On diagrams you can see there is still a small current through red LED, when the pin is in up state.

Is there any ideas how to deal with this current?

Answer 1

Two words - source, sink.

The output pin of an arduino is capable of sourcing (supplying an output current) as well as sinking (taking a current into the output and down to ground.) see http://arduino.cc/en/Tutorial/DigitalPins

You don't need any transistors to switch the LEDs.

In this circuit when the output is LOW (near 0V) the green LED will be ON, red is OFF. When the output is HIGH (near 5V) the red LED is ON , green is OFF.

The resistors limit the sink/source current to a safe value.

Answer 2

For the sake of completeness: If you wanted to provide more current than your MCU can deliver, you can still use two transistors, but this time one NPN and one PNP. NPN in this case works as a non-inverting switch, PNP is inverting.

Note: This only works if your Drive voltage (MCU pin) goes all the way from 0 to 5V. Not a problem with the ATMega.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Looks like you have voltage leaking from the 5V through the second transistor base to the emittor, which is giving you a faint glow (Similar to the problem I faced in this post). As the current from the Arduino is enough to power 1 LED at a time (with a ~300-470Ohm resistor), you could just use the circuit from my post to make this all work. Saves you a transistor :)

EDIT: The full circuit you should implement (your Arduino pin goes where IN1 is, and you should replace the 9V source with your 5V source or with another pin from the arduino) is this (from this post):

^{simulate this circuit – Schematic created using CircuitLab}

NOTE that this assumes LED1 is Red and LED2 is green - so you would have to invert your pin logic on the Arduino.

Answer 4

A good way to deal with this is to stop putting the LEDs in to the emitter part of the circuit. Instead put the LED's in the collector part of the circuit in series with the current limit resistors.

Another thing to consider is that if the MCU pin has adequate drive capability and a 5V voltage swing you may consider simply connecting up the LEDs as follows: