HC-SR04 is a commonly used module for non-contact distance measurement for distances from 2cm to 400cm. It uses sonar (like bats and dolphins) to measure distance with high accuracy and stable readings. It consists of an ultrasonic transmitter, receiver and control circuit. The transmitter transmits short bursts which gets reflected by target and are picked up by the receiver. The time difference between transmission and reception of ultrasonic signals is calculated. Using the speed of sound and ‘Speed = Distance/Time' equation, the distance between the source and target can be easily calculated.
HC-SR04 ultrasonic distance sensor module has four pins:
- VCC – 5V, input power
- TRIG – Trigger Input
- ECHO – Echo Output
- GND – Ground
- VCC -> 5 volt pin on Pi
- Trig -> Trigger activates the sensor and connect to GIO output pin
- Echo -> Receives the signal, read by GPIO input pin
- GND -> GROUND PIN
Time taken by pulse is actually for to and from travel of ultrasonic signals, while we need only half of this. Therefore time is taken as time/2.
Distance = Speed * Time/2
Speed of sound at sea level = 343 m/s or 34300 cm/s
Thus, Distance = 17150 * Time (unit cm)
For accurate distance readings, the output can be calibrated using a ruler. In the below program a calibration of 0.5 cm is added.
import RPi.GPIO as GPIO #Import GPIO library import time #Import time library GPIO.setmode(GPIO.BCM) #Set GPIO pin numbering TRIG = 23 #Associate pin 23 to TRIG ECHO = 24 #Associate pin 24 to ECHO print "Distance measurement in progress" GPIO.setup(TRIG,GPIO.OUT) #Set pin as GPIO out GPIO.setup(ECHO,GPIO.IN) #Set pin as GPIO in while True: GPIO.output(TRIG, False) #Set TRIG as LOW print "Waitng For Sensor To Settle" time.sleep(2) #Delay of 2 seconds GPIO.output(TRIG, True) #Set TRIG as HIGH time.sleep(0.00001) #Delay of 0.00001 seconds GPIO.output(TRIG, False) #Set TRIG as LOW while GPIO.input(ECHO)==0: #Check whether the ECHO is LOW pulse_start = time.time() #Saves the last known time of LOW pulse while GPIO.input(ECHO)==1: #Check whether the ECHO is HIGH pulse_end = time.time() #Saves the last known time of HIGH pulse pulse_duration = pulse_end - pulse_start #Get pulse duration to a variable distance = pulse_duration * 17150 #Multiply pulse duration by 17150 to get distance distance = round(distance, 2) #Round to two decimal points if distance > 2 and distance < 400: #Check whether the distance is within range print "Distance:",distance - 0.5,"cm" #Print distance with 0.5 cm calibration else: print "Out Of Range" #display out of range
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