8 replies to this topic

[Traveling Tech Guy]

Sorry if this comes across as a newbie question.
I just recently got (back) into electronics. I managed to put out my first Netduino project, that simulates a traffic light (yay ).

While buying LEDs at my local Fry's, I accidentally picked up a 2-color LED. It says that a different color will be shown if polarity is reversed. I used my multimeter in diode-test mode, and sure enough, I got a green color when I switched the probes.

Now the question is, how to do this programmatically. Suppose I have one of the LED legs connected to D13 and the other to GND.
I can easily light the LED by using an OutputPort, but how can I reverse it's polarity? All I can write to the port is 'true or 'false'.

Do I need another electric component to reverse the polarity, or is it possible to achieve with code only?
Do I use an analog pin? What am I missing?

Thanks for your time.
Guy

[Stefan]

you could use the built-in tristateport class on two pins. it can be set off, input and output.

[Dan Morphis]

I can easily light the LED by using an OutputPort, but how can I reverse it's polarity? All I can write to the port is 'true or 'false'.

Do I need another electric component to reverse the polarity, or is it possible to achieve with code only?

Easy :-) You use two ports, one wired to each leg. Write true to pin and false to pin 2 to get green, and reverse it for red. Make sure you use a current limiting resistor on one of the legs.

[Traveling Tech Guy]

Thanks Dan! It is easy (took 3 minutes to write code and verify) Stefan, where can i learn about that tristateport class? Sounds like a better way to use this LED in the future than what I'm doing right now. Thank you both. I love easy answers

[Mario Vernari]

Hi Guy, unfortunately you cannot use the tristate port to drive a bi-color led. The simplest way is as Dan explained. Two ports are the very minimum, because you need three states: green, red and off. You MAY use a tristate port, but only adding some hardware. Polarity apart, I suggest always to add a simple transistor to increase the led current. Even the MCU is robust enough, I'd prefer to stress it as few as possible. Cheers

[Omar (OZ)]

Hi Guy, unfortunately you cannot use the tristate port to drive a bi-color led.
The simplest way is as Dan explained.
Two ports are the very minimum, because you need three states: green, red and off.
You MAY use a tristate port, but only adding some hardware.

Polarity apart, I suggest always to add a simple transistor to increase the led current. Even the MCU is robust enough, I'd prefer to stress it as few as possible.
Cheers

I believe he can actually. If he sets one port to input and the other to true, the lights will be off (both). Tristate ports have the ability of a output and input port. You can set a tristate port to true, false, or input. I don't have any of those LEDs so i can't test it per se, but I've done pretty much the same thing in my Charlieplexing post. You are correct, 2 ports is the minimum.

Here you can find a tutorial on the TristatePort class, http://forums.netdui...tateport-class/

[Stefan]

I believe he can actually. If he sets one port to input and the other to true, the lights will be off (both). Tristate ports have the ability of a output and input port. You can set a tristate port to true, false, or input. I don't have any of those LEDs so i can't test it per se, but I've done pretty much the same thing in my Charlieplexing post. You are correct, 2 ports is the minimum.

That was exactly my idea
Haven't got the mats to test it right now since I'm at a weekend trip with my family, but in theory thas was what I was thinking of.

[Mario Vernari]

Omar, I meant only *one* (tristate) port is not enough. It has exactly three states, but its driver cannot used to drive a led in the desired manner. Of course a tristate can be used as a normal push-pull port, but he needs at least two pins. However, I attached a schematic about the simplest way to drive a bi-color led using just one 3-state port. If the port is a PWM also, it is possible to create several shades only adjusting the duty-cycle. Fault: there is no a decent current driver such a transitor, so that the current will be very small. Cheers

Attached Files

•  bi-color-drv.jpg   18.58KB   41 downloads

[Omar (OZ)]

Omar, I meant only *one* (tristate) port is not enough. It has exactly three states, but its driver cannot used to drive a led in the desired manner.
Of course a tristate can be used as a normal push-pull port, but he needs at least two pins.

However, I attached a schematic about the simplest way to drive a bi-color led using just one 3-state port.
If the port is a PWM also, it is possible to create several shades only adjusting the duty-cycle.
Fault: there is no a decent current driver such a transitor, so that the current will be very small.
Cheers

ah gotcha