ZachLabs Arduino

Lab 12: Ultrasonic Rangefinder

The ultrasonic rangefinder in your kit uses ultrasonic sound (sound frequencies that are too high to be heard by humans) to measure the distance to the nearest obstacle and convert it into a signal that can be read by a microcontroller. This is similar to how bats use echolocation to “see” in the dark.

Example Program

Connect the ultrasonic rangefinder to your Arduino by making the following connections with the long M-to-F jumper wires included in your kit, and then upload the program to your Arduino:

Ultrasonic Rangefinder	Arduino Pin
Vcc	5V
Trig	2
Echo	3
Gnd	GND

long readDistance(int trigPin, int echoPin)
{
    // Pulse the TRIG pin for 10 microseconds:
    digitalWrite(trigPin, HIGH);
    delayMicroseconds(10);
    digitalWrite(trigPin, LOW);

    // Measure the length of the pulse on the ECHO pin with a 24 ms timeout:
    return pulseIn(echoPin, HIGH, 24000);
}

void setup() 
{
    Serial.begin(9600);
    pinMode(2, OUTPUT);
    pinMode(3, INPUT);
}

void loop() 
{
    Serial.println(readDistance(2, 3));
}

If you open the serial monitor in the Arduino IDE you should see a stream of numbers being printed to the screen that get lower the closer you move an obstacle (like your hand or a book) to the front of the ultrasonic rangefinder.

Learn More

• The ultrasonic rangefinder sends out an ultrasonic pulse when it receives a HIGH signal on its trigger (“TRIG”) pin for at least 10 µs (microseconds). It then makes its “ECHO” pin go HIGH until the sound bounces back and is detected by the sensor, at which point it goes LOW and the measurement is complete.

• The function readDistance() in the example code above generates that 10 µs trigger pulse and then uses the built-in Arduino function pulseIn() to measure the length of the echo pulse (also in µs), which turns out to be directly proportional to the distance between the sensor and the nearest obstacle according to the speed of sound (343 m/s in dry, room-temperature air).

• The ultrasonic rangefinder in your kit is inexpensive, but also unreliable. When something goes wrong and the sensor does not "hear" the echo it will cause the pulseIn() function to time out and return 0. When the readDistance() function returns a value of 0 you should ignore it instead of treating it like a valid (and very short) measurement.

Assignment

Create a "personal space detector" with the ultrasonic rangefinder and three LEDs (green, yellow, and red) that is normally green but switches to yellow when someone walks within 2 meters of the sensor and red when someone walks within 1 meter of the sensor. Remember that measurements of 0 indicate an error and should be ignored. If your lights flicker too much, try adding a delay between successful measurements.

The readDistance() function returns the total time, in µs, for the ultrasonic sound to leave the rangefinder's speaker, bounce off the target, and return to the rangefinder's microphone. Although the speed of sound varies based on temperature and humidity, we can assume a standard speed of 343 m/s. This means that a measurement of 1000 µs corresponds to a total travel distance of 34.3 cm (to the target and back), or a distance to the target of 17.15 cm (half the total travel distance).