7.8. Parking Sensor
In this example we will use HC-SR04 ultrasonic distance sensor combined with a buzzer to simulate a car parking sensor system where the buzzer's beeping rate increases as objects get closer and closer.
At the end of this example you will learn:
- How to play sound through a buzzer
- How to map values from one range to another
- How to use
timemodule to control beeping duration
Parts required:
- Soldered NULA Mini Board
- Breadboard
- HC-SR04 Ultrasonic distance sensor
- Buzzer
- 1 x Qwiic cable
- Some jumper wires
Putting the components together
- Place the buzzer on the breadboard, connect the positive pin on GPIO 5, and other pin to GND.
- Connecting the Ultrasonic sensor is pretty simple and straightforward, just plug in one end of Qwiic cable in the NULA Mini board, and the other end in the sensor itself.
- If you have the native version (no Qwiic connector), follow the table below for connections:
| Ultrasonic sensor | NULA Mini |
|---|---|
| VCC | VCC (5V) |
| GND | GND |
| ECHO | IO18 (any GPIO) |
| TRIG | IO19 (any GPIO) |
Code Example
Import needed modules for Ultrasonic sensor along with Pin and PWM for the buzzer.
from UltrasonicSensor import UltrasonicSensor
from machine import I2C, Pin, PWM
import time
Here are defined constants that are used for controlling the sound behaviour. They specify when the sound starts / stops playing based on distance, and how fast / slow are the 'beeps' being played based on distance.
CONTINUOUS_THRESHOLD_CM = 35 # Below this threshold - play continous tone
BEEP_START_DIST_CM = 170 # No sound played if measured distance is beyond this value
MIN_BEEP_PERIOD_MS = 60 # Shortest beep cycle (fastest beeping rate)
MAX_BEEP_PERIOD_MS = 700 # Longest beep cycle (slowest beeping rate)
MAX_BEEP_ON_TIME_MS = 50 # Max play time for each 'beep'
Buzzer is connected to pin 5 on microcontroller and it will be driven at a frequency of 2000 Hz (this is the pitch). Duty cycle sets the loudness (higher = louder).
BUZZ_PIN = 5
BUZZ_FREQ_HZ = 2000
BUZZ_DUTY = 30000
buzz_pwm = PWM(Pin(BUZZ_PIN))
buzz_pwm.freq(BUZZ_FREQ_HZ)
buzz_pwm.duty_u16(0)
Create an ultrasonic sensor object that communicates via I2C (Qwiic mode).
i2c = I2C(0, scl=Pin(7), sda=Pin(6))
sensor = UltrasonicSensor(i2c)
# If you are using native version of the sensor
# sensor = UltrasonicSensor(trig_pin=19, echo_pin=18)
Variables to control timing and state for how often to measure distance and how the buzzer will behave.
MEAS_PERIOD_MS = 80 # Value that sets how often to take distance measurements
last_meas = time.ticks_ms() # Store timestamp of last distance measurement
last_toggle = time.ticks_ms() # Track timestamp of last time buzzer changed state
beep_on = False # Flag to save buzzer state
period_ms = -1 # Sound behavior : -1 = silent, 0 = continuous, > 0 = beep period
BEEP_ON_TIME_MS = 0 # Duration of play time (ON time) of each beep
Mapping function to convert a value from one range to another. In this case, we are mapping distance value to a beep period.
# Map function used to map distance to beep period
def map_distance_value(value, in_min, in_max, out_min, out_max):
if value < in_min:
value = in_min
elif value > in_max:
value = in_max
return int((value - in_min) * (out_max - out_min) / (in_max - in_min) + out_min)
Main loop that constantly takes new measures, and plays the buzzer tone accordingly. Based on how close the object is, set one out of 3 sound modes (CONTINUOUS, BEEPING, SILENT). While 'beeping' map the measured distance value to a period_ms (beep period) and set how long each beep stays on: BEEP_ON_TIME_MS. The closer the object, the faster the beeping sound.
# Main loop
while True:
# Store current 'clock' time
now = time.ticks_ms()
# ticks_diff() used for measuring difference between 2 ticks
# If 80 ms passed -> get reading & play sound
if time.ticks_diff(now, last_meas) >= MEAS_PERIOD_MS:
sensor.takeMeasure()
last_meas = now # Store the time of measurement
distance = sensor.getDistance() # Get distance from sensor
# Choose 'sound mode' based on measured distance
if distance <= CONTINUOUS_THRESHOLD_CM:
period_ms = 0 # Set to continuous
elif distance >= BEEP_START_DIST_CM:
period_ms = -1 # Set to silent
else:
# Map distance reading from [35, 200] to a beep period [60, 700], store in period_ms
period_ms = map_distance_value(distance, CONTINUOUS_THRESHOLD_CM, BEEP_START_DIST_CM, MIN_BEEP_PERIOD_MS, MAX_BEEP_PERIOD_MS)
# Define duration of the 'ON time' of each beep, make the beep last a quarter of period cycle,
# always between 10 and 50 milliseconds
on_time = int(period_ms / 4)
if on_time < 10:
on_time = 10
elif on_time > MAX_BEEP_ON_TIME_MS:
on_time = MAX_BEEP_ON_TIME_MS
BEEP_ON_TIME_MS = on_time
# Based on period_ms, turn the buzzer ON or OFF depending on mode
# Continuous mode
if period_ms == 0:
if not beep_on: # If the buzzer isn't ON
# Play continuous sound and keep it turned ON
buzz_pwm.duty_u16(BUZZ_DUTY)
beep_on = True
# Silent mode
elif period_ms == -1:
if beep_on: # If the buzzer is ON
# Turn off buzzer
buzz_pwm.duty_u16(0)
beep_on = False
# Beeping mode
else:
if not beep_on: # Currently OFF
# Wait for 'silent' part to finish (period_ms - BEEP_ON_TIME_MS), then turn it ON
if time.ticks_diff(now, last_toggle) >= (period_ms - BEEP_ON_TIME_MS):
buzz_pwm.duty_u16(BUZZ_DUTY) # Turn ON buzzer
beep_on = True # Change state
last_toggle = now # Track toggle time
else: # Currently ON
# Wait until 'ON-time (BEEP_ON_TIME_MS)' has passed, then turn if OFF
if time.ticks_diff(now, last_toggle) >= BEEP_ON_TIME_MS:
buzz_pwm.duty_u16(0) # Turn OFF buzzer
beep_on = False # Change state
last_toggle = now # Track toggle time