Utilising the IMP LEDs on IMP002

Hi all

I’m planning to integrate an IMP002 into a project, and I’d like a really minimal display - I’m thinking a single RGB LED, which would encompass my functionality and the built in IMP red & green LED functionality as well.

So essentially I’m asking how you would set up (in hardware) an LED to be controlled by more than one pin. I’m thinking that simply connecting both pins to the +ve terminal of the LED would not be the way to go… :wink:

diodes are your friend :slight_smile:

cool, I was hoping it would be that simple!

I’ve been thinking a bit more about this. I initially thought the suggestion was to just connect the IMP LED pin (eg Green) to the LED via a diode, and connect a second IO pin to the LED via a diode as well. So both pins can independently turn the LED on or off, without this other pin sourcing any current from it.

This raised a couple of questions though. I’m not sure the IO pins can source enough current for a decent brightness LED. The Red and Green pins are up to 20ma, but the spec for the other IO pins is just 4ma. Secondly, adding in the additional diode would increase the voltage drop, which would be fine for the Red LED (as the drop across this is low) but for the Green, the combined drops would go close to or over 3.3v.

So my thought was to use these pins with diodes, as before, but drive a transistor, meaning the LED can source more current directly from the 3.3v rail, and there wouldn’t be the additional diode in series with the LED, so voltage drop would be OK.

Does that make sense? Any reason why that wouldn’t work?


I haven’t played with the exact configuration you mention to drive a LED (i.e. drive the green segment between +3.3 and Vce-sat), but what you propose sounds sensible, and worth giving it a go. I can’t see any obvious reason why it shouldn’t fly. (I’ll just usually drive green/blue off +5 with a 220 ohm current-limiting R). I think your idea is worth playing around with a bit further.

Thanks - will give it a go!