varying control of leds

al cooke

I'm having trouble controlling brightness of led banks , 24 each RGB, each color connected in parallel. Hoped to use digital pots into op-amps - transistors/w feedback to vary incrementally. I'd appreciate any help - which op-amp, dig-pot, transistor tp use. Or where can I look for info? Using LM358, 2N2222, tried 10K and 100K pots, 5 & 12 volts but very little control between LED on - off. Thanks for any suggestions. Al

Vern Graner

al cooke said...
I'm having trouble controlling brightness of led banks , 24 each RGB, each color connected in parallel. Hoped to use digital pots into op-amps - transistors/w feedback to vary incrementally. I'd appreciate any help - which op-amp, dig-pot, transistor tp use. Or where can I look for info? Using LM358, 2N2222, tried 10K and 100K pots, 5 & 12 volts but very little control between LED on - off. Thanks for any suggestions. Al

It would appear from your description that you would be using a "resistive" approach to dimming the LEDs. If this is the case it's going to be much less efficient (i.e. turning a lot of energy into heat) than using Pulse Width Modulation. There was a posting to the ParallaxEFX board a few days back that detailed using PWM code to vary the brightness of LEDS:

http://forums.parallax.com/showthread.php?p=591981

Hope this helps [noparse]:)[/noparse]

Vern

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al cooke

Thanks for the reference Vern. I think you are right about PWM. Al Cooke