In order to use servos, we take advantage of pulseio. Now, in theory, you could just use the raw pulseio calls to set the frequency to 50 Hz and then set the pulse widths. But we would rather make it a little more elegant and easy!
So, instead we will use adafruit_motor which manages servos for you quite nicely! adafruit_motor is a library so be sure to grab it from the library bundle if you have not yet! If you need help installing the library, check out the CircuitPython Libraries page.
Servos come in two types:
- A standard hobby servo - the horn moves 180 degrees (90 degrees in each direction from zero degrees).
- A continuous servo - the horn moves 180 degrees like a DC motor. Instead of an angle specified, you set a throttle value with 1.0 being full forward, 0.5 being half forward, 0 being stopped, and -1 being full reverse, with other values between.
The connections for a servo are the same for standard servos and continuous rotation servos.
Connect the servo's brown or black ground wire to ground on the CircuitPython board.
Connect the servo's red power wire to 5V power, USB power is good for a servo or two. For more than that, you'll need an external battery pack. Do not use 3.3V for powering a servo!
Connect the servo's yellow or white signal wire to the control/data pin, in this case A1 or A2 but you can use any PWM-capable pin.
Standard Servo Code
Here's an example that will sweep a servo connected to pin A2 from 0 degrees to 180 degrees (-90 to 90 degrees) and back:
import time
import board
import pulseio
from adafruit_motor import servo
# create a PWMOut object on Pin A2.
pwm = pulseio.PWMOut(board.A2, duty_cycle=2 ** 15, frequency=50)
# Create a servo object, my_servo.
my_servo = servo.Servo(pwm)
while True:
for angle in range(0, 180, 5): # 0 - 180 degrees, 5 degrees at a time.
my_servo.angle = angle
time.sleep(0.05)
for angle in range(180, 0, -5): # 180 - 0 degrees, 5 degrees at a time.
my_servo.angle = angle
time.sleep(0.05)
Continuous Servo Code
There are two differences with Continuous Servos vs. Standard Servos:
- The
servoobject is created likemy_servo = servo.ContinuousServo(pwm)instead ofmy_servo = servo.Servo(pwm) - Instead of using
myservo.angle, you usemy_servo.throttleusing a throttle value from 1.0 (full on) to 0.0 (stopped) to -1.0 (full reverse). Any number between would be a partial speed forward (positive) or reverse (negative). This is very similar to standard DC motor control with the adafruit_motor library.
This example runs full forward for 2 seconds, stops for 2 seconds, runs full reverse for 2 seconds, then stops for 4 seconds.
# Continuous Servo Test Program for CircuitPython
import time
import board
import pulseio
from adafruit_motor import servo
# create a PWMOut object on Pin A2.
pwm = pulseio.PWMOut(board.A2, frequency=50)
# Create a servo object, my_servo.
my_servo = servo.ContinuousServo(pwm)
while True:
print("forward")
my_servo.throttle = 1.0
time.sleep(2.0)
print("stop")
my_servo.throttle = 0.0
time.sleep(2.0)
print("reverse")
my_servo.throttle = -1.0
time.sleep(2.0)
print("stop")
my_servo.throttle = 0.0
time.sleep(4.0)
Pretty simple!
Note that we assume that 0 degrees is 0.5ms and 180 degrees is a pulse width of 2.5ms. That's a bit wider than the official 1-2ms pulse widths. If you have a servo that has a different range you can initialize the servo object with a different min_pulse and max_pulse. For example:
servo = adafruit_motor.servo.Servo(pwm, min_pulse = 0.5, max_pulse = 2.5)
For more detailed information on using servos with CircuitPython, check out the CircuitPython section of the servo guide!
