Used to drive an analog RGB LED Strip using a raspberry pi and adafruit's 16-Channel 12-bit PWM/Servo Driver
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LED_Strip_bb.png Make the image a lot smaller Mar 28, 2013 Update Mar 29, 2013


This software can be used to drive an analog RGB LED strip using a raspberry pi and adafruit's 16-channel 12-bit PWM/Servo Driver - PCA9685:

The Circuit:

Here's the basic idea:

  • Hook up the pi to the PCA9685 breakout board using the I2C connections.
  • Connect the pi's 3.3V output to VCC on the PCA9685 breakout board. Leave V+ floating.
  • Follow this tutorial for the RGB LED strips:
    • I used the N-channel MOSFETs - three of them, one for each channel
    • Connect the +12V from the LED strip to an external power supply (do NOT use your pi for this!)
    • Connect the ground side of the power supply to the pi ground
    • Instead of using the PWM outputs from the arduino, we'll use the PWM outputs from the PCA9685.
    • Connect up the PWM output 0 to the MOSFET with the red wire from the LED strip.
      Output 1 goes to green, output 2 goes to blue.

Here's a picture


Adafruit's PWM Servo Driver software (Adafruit_PWM_Servo_Driver) must be in your PYTHONPATH. That software imports Adafruit_I2C, which should be in the same directory


This program is designed to be used from the command line. You should be able to see most options by typing:

./ --help
usage: [-h] [-r RED RED] [-g GREEN GREEN] [-b BLUE BLUE]
                     [-s STEPS] [-d DELAY] [-o]
                     [--red-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}]
                     [--green-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}]
                     [--blue-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}]
                     [--repeat REPEAT] [--reverse] [--random] [--time TIME]
                     [--max-random-walk MAX_RANDOM_WALK]

drive a rgb led strip through a pwm module

optional arguments:
  -h, --help            show this help message and exit
  -r RED RED, --red RED RED
                        The beginning and end values for red in fade mode. Max
                        and min values for red in random mode.
                        The beginning and end values for green in fade mode.
                        Max and min values for green in random mode..
  -b BLUE BLUE, --blue BLUE BLUE
                        The beginning and end values for blue in fade mode.
                        Max and min values for blue in random mode..
  -s STEPS, --steps STEPS
                        Number of steps in the fade. Not used with --random
  -d DELAY, --delay DELAY
                        Number of seconds between the steps or random changes,
                        can be a float
  -o, --turn-off        Turn off when the fade or random event is over.
  --red-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}
                        The red pwm pin
  --green-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}
                        The green pwm pin
  --blue-pin {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}
                        The blue pwm pin
  --repeat REPEAT       Repeat the fade this many times. Unused with --random.
  --reverse             Reverse the fade, to return back to the initial state.
                        Unused with --random.
  --random              Move around randomly between the max and min values
                        specified with --red --green and --blue. Use with
                        --time and --max-random-walk
  --time TIME           Used with --random. Move randomly for this many
  --max-random-walk MAX_RANDOM_WALK
                        The max that each channel will be allowed to change
                        between steps in random mode.

NOTE: You will likely need to run this as root or use sudo because of the I2C interface, unless you jump through a bunch more hoops. Using sudo can be tricky because of the PYTHONPATH. You can do something like this, though:

    sudo PYTHONPATH=$PYTHONPATH:/path/to/Adafruit_PWM_Servo_Driver ./ -r 0 4095 --repeat 3 -o

Alternatively, you could just copy into the Adafruit_PWM_Servo_Driver directory to save yourself some grief.

There are two modes:

  1. A fade from one color to another (and, optionally back again with --reverse).
    This can be repeated any number of times using --repeat. You can change the speed of the fade by modifying --delay and --steps.

  2. Random color changes within bounds and at a rate defined by --delay. The color changes a maximum of --max-random-walk per change. It will continue to change for approximately --time seconds.

Of course, you can also import the module and use random_walk and fade_rgb directly.


  1. Fade from off to max red, then turn off:

     sudo ./ -r 0 4095 -o
  2. Fade from off to max red and back and repeat this 3 times, turning off at the end:

     sudo ./ -r 0 4095 -o --repeat 3 --reverse
  3. Fade slowly from red to blue:

     sudo ./ -r 4095 0 -b 0 4095 -o -d .1 -s 100
  4. A random twilightish twinkle for 20s:

     sudo ./ -r 500 1000 -g 1024 2048 -b 2048 4095 -s 100 -d 0.1 -o --random --max-random-walk 100 --time 20
  5. Fireplace!

     sudo ./ -r 2000 4095 -g 0 1024 -b 0 0 -s 100 -d 0.1 -o --random --max-random-walk 100 --time 20
  6. Seizure!

     sudo ./ -r 0 4095 -g 0 4095 -b 4095 0  --delay 0.01  -o --random --max-random-walk 2000 --time 10

Here's a video showing these examples in order

What's Next?

The PWM breakout board has 16 outputs, so, with the right power supply, we could drive up to 5 strips at the same time.

I plan on updating the code to allow for driving multiple strips.