I am playing with the device from the Arduino ReadASCIIString tutorial. All is fine except that the colours displayed bear no resemblance (except for the three basic colours) to those represented by the number sequences I get from an RGB Colour Chart.
Is there any obvious reason why the colours do not correspond?
I have been toiling with this. The first thing I discovered is that I had the colour pins for the LED wrong. With the flat to my right (5 mm Common Anode LED) I expected the pins to be RAGB - instead they are RABG - so I swapped the pins Now, at least, the primary colours are correct, but I still have mixing problems.
I started to suspect the relative intensities of the individual colours in the LED. Checking a tutorial that deals with loading resistors for LEDs I discover the formula
R=(Vs - Vf)/If
Vs is the source voltage – usually a battery or power supply voltage. Vf and If are the LED’s forward voltage and the desired current that runs through it. This gives me (according to my always suspect mathematics) a value of 32 ohms for the resistor for Red. That seems incredibly low (I am using 10K).
Question: What does the value given by this formula actually represent? Is it a resistance which will stop me frying (another) LED? Is it a resistance which will produce maximum luminosity from the LED? What DOES this value represent?
Question: Given that the datasheet for the LED tells me
Luminous Intensity (mcd): 500 (Red) 1200 (Green) 600 (Blue)
how do I balance the colour intensity for each primary? Do I use different loading resistors for each cathode? If so, how do I calculate appropriate resistance values?
'Scuse me for being a bit of a pain, but I would like to get to the bottom of this. The code I am using is
// pins for the LEDs:
#define RED_PIN 3
#define GREEN_PIN 5
#define BLUE_PIN 6
// variable
bool debug = true;
void setup()
{
// initialize serial I/O
Serial.begin(9600);
// initialize pins
pinMode(RED_PIN, OUTPUT);
pinMode(GREEN_PIN, OUTPUT);
pinMode(BLUE_PIN, OUTPUT);
}
void loop()
{
// if there's any serial input available, read it
while (Serial.available() > 0)
{
// note something is read
if (debug)
{
Serial.println();
Serial.println("Something has been read");
}
// look for the next valid integer in the incoming serial stream
int red = Serial.parseInt();
if (debug)
{
Serial.print("Red Value => ");
Serial.println(red);
}
// do it again
int green = Serial.parseInt();
if (debug)
{
Serial.print("Green Value => ");
Serial.println(green);
}
// do it again
int blue = Serial.parseInt();
if (debug)
{
Serial.print("Blue Value => ");
Serial.println(blue);
Serial.println();
}
// look for the newline. That's the end of your sentence
// Note: to do this you must adjust the settings in your Serial
// Monitor to add the newline character when 'Enter' is pressed.
// It is not there by default.
if (Serial.read() == '\n')
{
// constrain entered integers to ASCII range
red = constrain(red, 0, 255);
green = constrain(green, 0, 255);
blue = constrain(blue, 0, 255);
if (debug)
{
Serial.print("Red Value constrained to => ");
Serial.println(red);
Serial.print("Green Value constrained to => ");
Serial.println(green);
Serial.print("Blue Value constrained to => ");
Serial.println(blue);
Serial.println();
}
// because I am using a "Common Anode" LED, I get bright colour
// when pin value is low, so I have to reverse the values
// entered
red = 255 - red;
green = 255 - green;
blue = 255 - blue;
if (debug)
{
Serial.print("Final Red Value => ");
Serial.println(red);
Serial.print("Final Green Value => ");
Serial.println(green);
Serial.print("Final Blue Value => ");
Serial.println(blue);
Serial.println();
}
// write the colour values to respective pins
analogWrite(RED_PIN, red);
analogWrite(GREEN_PIN, green);
analogWrite(BLUE_PIN, blue);
// print the three numbers in hexadecimal
if (debug)
{
Serial.print(red, HEX);
Serial.print(", ");
Serial.print(green, HEX);
Serial.print(", ");
Serial.println(blue, HEX);
Serial.println();
}
}
}
}
The site you're getting RGB codes from appears to be for computer monitors. An RGB LED is going to mix a bit differently. And not all RGB LEDs are going to be equal in that regard.
I guess I should have known better, Delta_G 
- expecting standardization and rationalization in the computer world does ask a lot. :
Still, the questions occupying my mind at the moment are:
vagulus:
Question: What does the value given by this formula actually represent? Is it a resistance which will stop me frying (another) LED? Is it a resistance which will produce maximum luminosity from the LED? What DOES this value represent?Question: Given that the datasheet for the LED tells me
Luminous Intensity (mcd): 500 (Red) 1200 (Green) 600 (Blue)
how do I balance the colour intensity for each primary? Do I use different loading resistors for each cathode? If so, how do I calculate appropriate resistance values?
All I get in the way of mixed colour at the moment is a limited range of greens and an equally limited range of blues - mostly pale and insipid. That tells me that the intensity of the Red is being overpowered by the other two Primary Colours (which would be consistent with their candela ratings). I am looking for a way to dim Green and Blue. I guess changing the loading resistors might be the key - and I am going to play with that - but I am hoping that there is a calculation to tell me the resistor values to use.