Day 12: (Mon Oct 5, Week 7) Hobby Servos and Photointerrupters 2

Assignments

Due Wed Oct 7: Exercise: Autonomous Toy. Please have the device ready to demo in class and submit documentation to Assignment 6 Shared Folder. Please note this is the last deadline before Fall Break, so there can be no extensions.

Agenda

1. Review results of Exercise: Pico Music. Please remember to submit documentation to Assignment 5 Shared Folder.

2. Show a sample solution for Exercise: Autonomous Toy.

   • Pico Servo with Photointerrupter (circuit example)

   • Lite-On LTH-1550 Reflective Photointerrupter (part reference)

3. Individual consultation.

CAD for Sample Solution

Code for Sample Solution

The following sample uses the servo.py module documented under Servo Module, which should be copied to the top-level CIRCUITPY directory

# servo_balancer.py
# Uses a hobby servo to balance a marble on a tilting track using photosensor feedback.
#
# This assumes a tiny 9G servo has been wired up to the Pico as follows:
#   Pico pin 40 (VBUS)  -> servo red   (+5V)
#   Pico pin 38 (GND)   -> servo brown (GND)
#   Pico pin 1  (GP0)   -> servo orange (SIG)
#
# This assumes a LTH-1550 photoreflective sensor is connected to digital input
# GP15 using an external 22K pullup to 3.3V on GP15 and the collector lead of the
# phototransistor.  The sensor is active-low: the input is false when an object
# is present.
#   Pico pin 20 (GP15)  <- photoreflective sensor

################################################################################
# load standard Python modules
import math, time

# load the CircuitPython hardware definition module for pin definitions
import board

# load the module for the sensor input
from digitalio import DigitalInOut, Direction, Pull

# load the servo.py module located in same folder
import servo

#--------------------------------------------------------------------------------
# Set up a digital sensor input on GP15, which is physically pin 20 at one corner of the board.
# E.g., this may be attached photointerrupter with associated pullup resistor.
sensor = DigitalInOut(board.GP15)
sensor.direction = Direction.INPUT

# Block until an object is present at the sensor.
def wait_until_present():
    while True:
        if sensor.value is False:
            print("detected object")
            return
        time.sleep(0.01)

#--------------------------------------------------------------------------------
# Set up the servo object on GP0.
actuator = servo.Servo(pin=board.GP0)

#--------------------------------------------------------------------------------
# Calibration constants empirically tuned for the specific device.
center = 78
half   = 10
delay  = 0.1

print("moving to center")
actuator.write(center)
time.sleep(1.0)

#--------------------------------------------------------------------------------
# Begin the main processing loop.
while True:

    print("moving to low position")
    actuator.write(center-half)
    wait_until_present()
    time.sleep(delay)

    print("moving to high position")
    actuator.write(center+half)
    wait_until_present()
    time.sleep(delay)