Analog RGB LED strip brightness

EL Wire/Tape/Panels, LEDs, pixels and strips, LCDs and TFTs, etc products from Adafruit

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DropbearNinja
 
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Joined: Wed Apr 09, 2014 9:08 pm

Analog RGB LED strip brightness

Post by DropbearNinja »

Hi, I want to make this adafruit project: https://learn.adafruit.com/rgb-led-strips but I'd like to be able to adjust the overall brightness of the leds. For simplicity I was hoping to just use voltage to do so.

Looking at the usage page, could I simply use a 4th MOSFET and 4th PWM from arduino to MOSFET gate, +12V from PSU (NOT arduino) to drain, and source to the LED trip +12V. That way adjusting the duty cycle would adjust the voltage going to to the led strip.

Is this correct?

If so, would I be able to visually see the 'blinking' from the PWM controlling the power? Would I need to use a simple PWM->DAC filter like this: http://www.instructables.com/id/Analog- ... o-Voltage/

i.e. I could have arduino --> MOSFET --> PWM/DAC --> LED strip, controlling the arduino PWM duty cycle then gives fairly precise (steady) voltage control?

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adafruit_support_bill
 
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Joined: Sat Feb 07, 2009 10:11 am

Re: Analog RGB LED strip brightness

Post by adafruit_support_bill »

The N-channel MOSFETs from that tutorial are not sutable for "high-side" switching. You would need a P-channel device for that. Also, you would likely have problems with interference between the PWM rates of the high-side and the low-side. You would get annoying flashing at 'beat frequencies'. A simple RC filter as in the instructable you linked would not work well for the current-levels involved.

Overall, a software solution would be far simpler. Simply reduce the PWM duty cycle proportionally on all three channels.

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