Keyestudio Smart Little Turtle Robot V3 Manual page 100

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(8) Code Explanation
int trigPin- this pin is defined to transmit ultrasonic waves, generally
output.
int echoPin -
this is defined as the pin of reception, generally input
cm = (duration/2) / 29.1-
inches = (duration/2) / 74-
We can calculate the distance by using the following formula:
distance = (traveltime/2) x speed of sound
The speed of sound is: 343m/s = 0.0343 cm/uS = 1/29.1 cm/uS
Or in inches: 13503.9in/s = 0.0135in/uS = 1/74in/uS
We need to divide the traveltime by 2 because we have to take into
account that the wave was sent, hit the object, and then returned back to
the sensor.
(9) Extension Practice:
We have just measured the distance displayed by the ultrasonic. How
about controlling the LED with the measured distance? Let's try it, connect
an LED light module to the D3 pin.
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