Python Usage

It's easy to control PWM or servos with the Adafruit 16-channel PWM/Servo HAT and Bonnet. There are multiple CircuitPython libraries available to work with the different features of these boards including Adafruit CircuitPython PCA9685, and Adafruit CircuitPython ServoKit. These libraries make it easy to write Python code to control PWM and servo motors.

Python Wiring

First assemble the HAT or Bonnet exactly as shown in the previous pages. There's no wiring needed to connect the HAT or Bonnet to the Pi. The example below shows the HAT attached to a Pi.

To dim an LED, wire it to the board as follows. Note: you don't need to use a resistor to limit current through the LED as the HAT and Bonnet will limit the current to around 10mA.

  • Connect LED cathode / shorter leg to HAT or Bonnet channel GND / ground.
  • Connect LED anode / longer leg to HAT or Bonnet channel PWM.

External power is not necessary to PWM an LED.

To control a servo, wire it to the board as shown in the previous pages, including a barrel jack to the power terminal to attach an appropriate external power source to the HAT or Bonnet. The HAT and Bonnet will not power servos without an external power source!

Python Installation of ServoKit Library

You'll need to install the Adafruit_Blinka library that provides the CircuitPython support in Python. This may also require enabling I2C on your platform and verifying you are running Python 3. Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!

Once that's done, from your command line run the following command:

  • sudo pip3 install adafruit-circuitpython-servokit

If your default Python is version 3 you may need to run 'pip' instead. Just make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!

Python Usage

To demonstrate the usage, we'll use Python code to control PWM to dim an LED and to control servo motors from the Python REPL.

Dimming LEDs

This HAT and Bonnet use the PCA9685. Each channel of the HAT and Bonnet can be used to control the brightness of an LED.  The PCA9685 generates a high-speed PWM signal which turns the LED on and off very quickly.  If the LED is turned on longer than turned off it will appear brighter to your eyes.

First you'll need to import the necessary modules, initialize the I2C bus for your board, and create an instance of the class.

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import board
import busio
import adafruit_pca9685
i2c = busio.I2C(board.SCL, board.SDA)
hat = adafruit_pca9685.PCA9685(i2c)

The PCA9685 class provides control of the PWM frequency and each channel's duty cycle.  Check out the PCA9685 class documentation for more details.

For dimming LEDs you typically don't need to use a fast PWM signal frequency and can set the board's PWM frequency to 60hz by setting the frequency attribute:

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hat.frequency = 60

The HAT and Bonnet support 16 separate channels that share a frequency but can have independent duty cycles. That way you could dim 16 LEDs separately!

The PCA9685 object has a channels attribute which has an object for each channel that can control the duty cycle. To get the individual channel use the [] to index into channels.

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led_channel = hat.channels[0]

Now control the LED brightness by controlling the duty cycle of the channel connected to the LED. The duty cycle value should be a 16-bit value, i.e. 0 to 0xffff (65535), which represents what percent of the time the signal is on vs. off.  A value of 0xffff is 100% brightness, 0 is 0% brightness, and in-between values go from 0% to 100% brightness.

For example set the LED completely on with a duty_cycle of 0xffff:

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led_channel.duty_cycle = 0xffff

After running the command above you should see the LED light up at full brightness!

Now turn the LED off with a duty_cycle of 0:

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led_channel.duty_cycle = 0

Try an in-between value like 1000:

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led_channel.duty_cycle = 1000

You should see the LED dimly lit.  Try experimenting with other duty cycle values to see how the LED changes brightness!

For example make the LED glow on and off by setting duty_cycle in a loop:

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# Increase brightness:
for i in range(0xffff):
    led_channel.duty_cycle = i

# Decrease brightness:
for i in range(0xffff, 0, -1):
    led_channel.duty_cycle = i

These for loops take a while because 16-bits is a lot of numbers. CTRL-C to stop the loop from running and return to the REPL.

That's all there is to dimming LEDs using CircuitPython and the PWM/Servo HAT and Bonnet!

Controlling Servos

We've written a handy CircuitPython library for the various PWM/Servo kits called Adafruit CircuitPython ServoKit that handles all the complicated setup for you. All you need to do is import the appropriate class from the library, and then all the features of that class are available for use. We're going to show you how to import the ServoKit class and use it to control servo motors with the Adafruit PWM/Servo HAT or Bonnet.

First you'll need to import and initialize the ServoKit class. You must specify the number of channels available on your board. The HAT and Bonnet have 16 channels, so when you create the class object, you will specify 16.

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from adafruit_servokit import ServoKit
kit = ServoKit(channels=16)

Now you're ready to control both standard and continuous rotation servos.

Standard Servos

To control a standard servo, you need to specify the channel the servo is connected to. You can then control movement by setting angle to the number of degrees.

For example to move the servo connected to channel 0 to 180 degrees:

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kit.servo[0].angle = 180

To return the servo to 0 degrees:

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kit.servo[0].angle = 0

With a standard servo, you specify the position as an angle. The angle will always be between 0 and the actuation range. The default is 180 degrees but your servo may have a smaller sweep. You can change the total angle by setting actuation_range.

For example, to set the actuation range to 160 degrees:

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kit.servo[0].actuation_range = 160

Often the range an individual servo recognises varies a bit from other servos. If the servo didn't sweep the full expected range, then try adjusting the minimum and maximum pulse widths using set_pulse_width_range(min_pulse, max_pulse).

To set the pulse width range to a minimum of 1000 and a maximum of 2000:

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kit.servo[0].set_pulse_width_range(1000, 2000)

That's all there is to controlling standard servos with the PWM/Servo HAT or Bonnet, Python and ServoKit!

Continuous Rotation Servos

To control a continuous rotation servo, you must specify the channel the servo is on. Then you can control movement using throttle.

For example, to start the continuous rotation servo connected to channel 1 to full throttle forwards:

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kit.continuous_servo[1].throttle = 1

To start the continuous rotation servo connected to channel 1 to full reverse throttle:

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kit.continuous_servo[1].throttle = -1

To set half throttle, use a decimal:

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kit.continuous_servo[1].throttle = 0.5

And, to stop continuous rotation servo movement set throttle to 0:

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kit.continuous_servo[1].throttle = 0

That's all there is to controlling continuous rotation servos with the PWM/Servo HAT or Bonnet, Python and ServoKit!

Full Example Code

"""Simple test for a standard servo on channel 0 and a continuous rotation servo on channel 1."""
import time
from adafruit_servokit import ServoKit

# Set channels to the number of servo channels on your kit.
# 8 for FeatherWing, 16 for Shield/HAT/Bonnet.
kit = ServoKit(channels=8)

kit.servo[0].angle = 180
kit.continuous_servo[1].throttle = 1
time.sleep(1)
kit.continuous_servo[1].throttle = -1
time.sleep(1)
kit.servo[0].angle = 0
kit.continuous_servo[1].throttle = 0
This guide was first published on Jan 02, 2015. It was last updated on Jan 02, 2015. This page (Python Usage) was last updated on Dec 05, 2019.