Pet Feeder – Position Sensor Tests

I worked on testing the sensor board. The first tests I did were about the position sensor.

Recap

Let me recap the design of the position sensor in the device: The turntable has five position slots, marked red in the following illustration.

Behind this slots, there is a small excavation, reflecting the light from the infrared LED to the phototransistor.

Circuit

The circuit, with the LED and the phototransistor is shown in the following circuit diagram:

There is a line POS-IN and POS-OUT connected to the microcontroller. The idea is to keep this circuit without power if it isn’t used. As soon 3.3V is applied to POS-IN, the infrared LED lights up, and the voltage divider between the resistor R14 and the phototransistor, gets a working voltage. Therefore, POS-OUT gets a voltage between VCC and GND.

I use an Everlight PT333-3B phototransistor, with a peak wavelength of 940nm and a matching Everlight IR333/H0/L10 LED, with the same wavelength.

More light on the phototransistor will conduct more current to GND, therefore the voltage will drop. So, the resistor R14 controls the sensitivity of this detector.

First Measurement

I started with a 10kΩ resistor, which was a too strong pull-up, so I changed it into a 100kΩ resistor. Here you can see the first measurement I did, rotating the turntable at a slow speed.

The signal is between 3.0V and 1.5V, which is perfect for the task. The response curve is very smooth, so it is easy to set a trigger point to get the correct timing for the rotation.

Controller Setup with MicroPython

I set up a microcontroller with a stepper driver and connected it to the motor and position sensor. As a microcontroller, I used a Raspberry Pi Pico. The stepper driver is not the final one, I need to test a few more, less noisy drivers. 😄

In MicroPython I wrote simple test code for sensor readout, interpolation and calibration:

from machine import Pin, PWM, ADC
import utime

stepper_sleep = Pin(13, Pin.OUT, value=0)
dir_pin = Pin(15, Pin.OUT, value=0)
step_pwm = PWM(Pin(14))

pos_sens_out = Pin(12, Pin.OUT, value=0)
pos_sens_in = ADC(26)


trigger_sensor_value = 0x9500


def read_pos_sensor() -> int:
    pos_sens_out.value(1)
    utime.sleep_ms(5)
    sensor_sum = 0
    sensor_samples = const(16)
    for _ in range(sensor_samples):
        sensor_sum += pos_sens_in.read_u16()
        utime.sleep_us(100)
    pos_sens_out.value(0)
    return sensor_sum // sensor_samples

def is_at_position() -> bool:
    return read_pos_sensor() < trigger_sensor_value

def start_turntable():
    stepper_sleep.value(1)
    utime.sleep_ms(10)
    dir_pin.value(0)
    utime.sleep_ms(10)
    step_pwm.duty_u16(0x8000)
    step_pwm.freq(500)
    for freq in range(500, 5000, 100):
        step_pwm.duty_u16(0)
        step_pwm.freq(freq)
        step_pwm.duty_u16(0x8000)
        utime.sleep_ms(10)
    step_pwm.freq(5000)
    
def stop_turntable():
    step_pwm.freq(100)
    step_pwm.duty_u16(0)
    utime.sleep_ms(10)
    dir_pin.value(0)
    stepper_sleep.value(0)
        
def move_turntable_one_slot():
    print('Start turntable')
    start_turntable()
    if is_at_position():
        print('It is at position, wait until we leave')
        while is_at_position():
            utime.sleep_ms(10)
    print('Wait for the next position')
    while not is_at_position():
        utime.sleep_ms(10)
    print('Stop the turntable')
    stop_turntable()

def test_speeds():
    stepper_sleep.value(1)
    utime.sleep_ms(10)
    dir_pin.value(0)
    utime.sleep_ms(10)
    for freq in range(100, 10000, 100):
        print(freq)
        step_pwm.duty_u16(0x8000)
        step_pwm.freq(freq)
        utime.sleep_ms(100)
    step_pwm.freq(100)
    step_pwm.duty_u16(0)
    utime.sleep_ms(10)
    dir_pin.value(0)
    stepper_sleep.value(0)
        
        
def calibrate_pos_sensor():
    print('Record max and min for 10 seconds')
    max_value = 0x0000
    min_value = 0xffff
    start_turntable()
    end_time = utime.ticks_add(utime.ticks_ms(), 10000)
    while utime.ticks_diff(utime.ticks_ms(), end_time) < 0:
        sensor_value = read_pos_sensor()
        max_value = max(max_value, sensor_value)
        min_value = min(min_value, sensor_value)
        utime.sleep_ms(50)
    print('Calibration finished.')
    print('Min: 0x{:04x} Max: 0x{:04x}'.format(min_value, max_value))
    print('Suggested trigger: 0x{:04x}'.format(min_value+(max_value-min_value)//2))
    stop_turntable()
    

move_turntable_one_slot()

Video

The following video shows the controller in action. The turntable precisely turns to the next slot. I expected I need to add calibration, where I can add a number of steps after the position sensor gets triggered. But the readout is clean enough, to work without additional movement. If I stop the motor exactly at the trigger point, the slot is perfectly aligned with the output funnel.

You can hear the original sound of the motor with the current stepper driver. It is way too loud for this device. Hopefully, drivers with higher PWM frequencies will drive the motor with less noise.

Conclusion and Prospects

The position sensor on the board works flawlessly and way better than expected. Next, I test the sensors for the compartments. Here, I expect I need some small changes on the lids, adding a white part for better reflection.

If these tests are successful, I will publish the next part in the pet-feeder series, with all details and sources about the sensor board.

I hope you found this update about the tests interesting. If you have any questions, missed information, or simply want to provide feedback, feel free to comment below or contact me on Twitter. 😄