Electronic – RGB LED brightness levels

ledraspberry pirgbtransistors

Do RGB LEDs have very different brightness levels?

I'm running an RGB LED (common cathode) via some TIP120 transistors, controlled from a raspberry Pi.

There is a 5v feed from the rPi.

Initially I had ~2k resistors in series with the output pins and the base of the trans. but they were failing to trigger so I've removed them.

I also then had what I thought were the correct resistor values in series with the LED (82R and 150R for red and blue / green respectively) but they were so dim as to be useless.

So those have gone as well.

So against everything I know to be "right" I've got no resistors on the LED or the output pins.
The red is nice and bright but green and blue are still both pretty dim.
I'm controlling with software PWM from the pi and have had to limit the red channel to 25% of the green and blue just to level things out a bit.

Any idea what's going on?

Best Answer

Without a schematic, based on your question, you have Common Cathode Led with TIP120 transistors? I assume you have them connected backwards from normal.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

The TIP120 is a NPN darlington pair transistor. It normally expects to be on the low side of the load. You are using them for High Side Switching, which won't work. If your led was common anode, you could swap them around, and it would work (WITH THE RESISTORS).

But in this case, you have 1) A different signal level from the led power level (3.3v vs 5v) so Direct PNP transistor won't work, and 2) The Raspberry PI GPIO cannot source a lot of current. 16mA at max. So You need both NPN and PNP transistors. Some 2n3904 and 2n3906 Common npn and pnp transistors, but any similar would work

schematic

^{simulate this circuit}

I only show the blueled (5v source voltage - 3.2v forward voltage - 0.2v VCE drop / 20mA = 80Ω), do the same for the green led, and use a 140Ω or higher resistor for the red led.

Related Solutions

Electronic – arduino – Anode RGB LED Arduino Hookup Confusion

I had the same issue trying to use RGB LED that came with my kit.

Indeed, all the examples for my kit were written for common-cathode RGB LEDs. The PWM output values in the code assume common-cathode, and the wiring diagrams have the non-RGB lead wired to GND. It took me some time to figure out that the LEDs I got were common-Anode, and that the non-RGB lead should be connected to VCC instead of GND.

In adapting your examples, try this trick of inverting the scale just as your write the value out onto the PWM pins.

 analogWrite(Red,255-x);
 analogWrite(Blu,255-y);
 analogWrite(Grn,255-z);

This nice thing about doing it that way is all the other example Arduino code out there written for common-cathode can be used basically as-is with just this final adjustment of the values written.

Electronic – arduino – RGB LED – Set intensity on Red and Blue

The problem ended up being a PWM issue. (It was solved in the comments, however I've posted the following to help future readers of this.)

First switch the Red or Blue with the Green. This will determine where the problem lies. If the Red or Blue still doesn't work then the problem is with the LED (very unlikely) or somewhere in the connection. Otherwise it is a software problem, like incorrect use of PWM.

Helpful links:

Best Answer

Related Solutions

Electronic – arduino – Anode RGB LED Arduino Hookup Confusion

Electronic – arduino – RGB LED – Set intensity on Red and Blue

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