RBG LED how to?

[madthug]

Greetings,
I am searching for methods of driving RGB LEDs with a microcontroller such as a basic stamp. I’d like to fashion the LEDs in an array capable of displaying a range of colors. Do any of you have experience with these things? Can RGB LEDs be controlled using PWM for each channel? Do they require unique circuitry?
Thanks,
MadThug
???

[audioguru]

Hi Madthug,
I have seen RGB arrays being driven by analog current changes for its brightness changes.
Are you planning to provide a unique PWM pulse width for each of the 3 colours for each pixel on the entire screen? ???

[madthug]

Audioguru,
Thanks for your reply. My initial idea was to use a unique PWM for each channel. This should work but to get white each channel will require a different duty cycle. I looked in to driver chips but haven’t found something simple enough yet. The problem is that I only have so many pins that I can use! So, are you aware of any pitfalls with RGB LEDs or should I just get one and start experimenting?
Thanks,
MadThug (Should change to Mad Geek)
:-X

[audioguru]

Hi MadThug,
You will need many RGB LEDs and might have a problem having them matched for tint and brightness. Agilent sells white LEDs with your choice of tint.

If you are making a digital TV screen with LEDs then each vertical column and horizontal row intersection would have the 3 colours and each pixel lights its 3 colours as required by the scanning. Since the duty-cycle for each pixel is very low due to the scanning, then the LED current must be massive in order to obtain sufficient brightness. Maybe RGB LEDs can't dissipate heat as well as separate LEDs and therefore won't be very bright at their max instantaneous current.

[MP]

madthug,
Here is a link where you might be able to get samples:

http://www.lc-led.com/Products/department/27?OVRAW=rgb%20led&OVKEY=rgb%20led&OVMTC=standard

You will need to form a matrix for the color switching with the micro or use a lot of pins.

MP

[Sukhbinder]

madthug,
if u want to use RGB or RBG leds remember that if u want white colour from them, u need to have equal intensities for each of the 3 colours. Keep in mind that different colour leds have different forward voltages which is minimum for red and maximum for blue. if to reduce the voltage u can control the intensity to an extent but i reccommend controlling the current through the led which is a lot more trickier. Generally u can use PWM methods to control the intensity but varying the average current through the device, but keep in mind that even i u reduce the led current to nearly half its value, the change in intensity is very small (they use this to reduce the power comsumption for battery driven LED indicators). ::)