I have searched google and I now know that if we change the resistance, we can change the brightness of an LED, but I was wondering how to change resistance by using a micro controller.

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    Normally you use PWM instead of messing with resistances. It's easier and more efficient. – Majenko Feb 8 '16 at 18:15
  • There are I2C controlled resistors. But I know none that can handle 20W. Most likely a SSR (Solid State Relay) and PWM'ing would work but I don't know how good idea that is. – Avamander Feb 8 '16 at 18:45
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    High power leds don't use resistors, but have special drivers that regulate the voltage going into the led. With streetlights these drivers also handle the AC-to-DC conversion. It's unlikely you can dim these externally. – Gerben Feb 8 '16 at 19:33
  • There are LED drivers that have external regulation options but as OP asked about changing resistance with a microcontroller the answer would be with a I2C pot, but it's unsuitable for dimming a 20W LED. – Avamander Feb 8 '16 at 19:52

Did you do a web search? "arduino pwm high power led driver" gives all the info you may need to answer this.

Here is a nice youtube clip, by Julian Ilett that explains how to do this: 60W High Power LED Driver with PWM #2 - Brightness Control.

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    Mikael, this is basically a link-only answer. Can you expand on it a bit please in case the link goes down? – Nick Gammon Feb 8 '16 at 20:36
  • The video is by Julian Ilett, who has a rather large collection of tutorials on YouTube, so hopefully the link won't go down. – Greenonline Feb 8 '16 at 21:37

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