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Ultrasonic Distance Testing

The HC-SR04 sensor provides distance measurements with reasonable accuracy within a certain range (2cm - 400cm).

Operation

It has 4 pins

  1. Vcc - 5V supply
  2. Trig - A signal sent to the sensor to trigger emmit and receive functions on the component
  3. Echo - Emits a signal that can be observed
  4. GND - Ground Connection

Example implementation

Components List

  1. 1 - Ultrasonic Sensor (HC-SR044)
  2. 4 - Female to male connectors

Note that Board numbering is used, NOT Broadcom numbering

  1. Pin 2 (5V Vcc)

  2. Pin 9 (GND)

  3. Pin 38 (Trig)

  4. Pin 40 (Echo)

GPIO Pinout

Refer to this diagram to understand the pins we are using

Note that Board numbering is used NOT Broadcom numbering

Diagram Schematic

Diagram Scematic

Diagram Schematic

Circuit Schematic

Circuit Schematic

Operation

Trigger Signal

The trigger signal prompts the sensor to emit ultrasonic waves, listen for them and indicate when the signal has been received. The characteristic of the signal is a prolonged low signal followed for a high signal maintained for 10µs, then a low signal until the next trigger signal is sent.

Echo Signal

After the trigger signal is sent, the sensor will emit a high signal on the echo channel after an undefined time. When the signal falls, it indicates the waves have made a round trip from the sensor to the target.

Calculating Distance

The speed of sound in air is 330m/s (at room temp). The round trip time should be divided into two then multiplied by the speed of sound to get distance.

ultrasonic_distance.py
# A program that measues the distance between the front of a HC_SRQ4 ultrasonic sensor to a target within range

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)

# Pins
echo = 40
trig = 38

GPIO.setup(trig, GPIO.OUT)
GPIO.setup(echo, GPIO.IN)

# Sends a trigger signal to the sensor requesting it to do it's magic
def sendTrigger():
  GPIO.output(trig, 0)
  time.sleep(2E-6)
  GPIO.output(trig, 1)
  time.sleep(10E-6)
  GPIO.output(trig, 0)

# Returns the time delta of a round trip time for the sensor's signal
def delta() -> float:
  while GPIO.input(echo) == 0:
    pass
  start = time.time()
  while GPIO.input(echo) == 1:
    pass
  end = time.time()

  # Converting from micro seconds to seconds
  return (end - start) * 1E6

# Returns the distance between the sensor and the object in centimeters
def distance(delta: float) ->float:
  # Halfing the time taken
  delta *= 0.5
  speed = 330 #  330m/s

  # Converting the distance to centimeters
  return (delta * speed) * 100

try: 
  while True:
    sendTrigger()
    print(distance(delta()), "cm")
    time.sleep(.5)

except KeyboardInterrupt:
  GPIO.cleanup()
  print("\nExiting...\n")

The code. link