DistanceSensor (ultrasonic) in Raspberry Pi - Step-by-Step Execution

Concept Flow - DistanceSensor (ultrasonic)

Start

↓

Send Trigger Pulse

↓

Wait for Echo Start

↓

Wait for Echo End

↓

Calculate Duration

↓

Calculate Distance

↓

Output Distance

↓

End

The sensor sends a pulse, waits for the echo, measures the time, then calculates and outputs the distance.

Execution Sample

Raspberry Pi

from gpiozero import DistanceSensor
sensor = DistanceSensor(echo=17, trigger=4)
print(sensor.distance * 100)

This code measures distance using an ultrasonic sensor and prints it in centimeters.

Execution Table

Step	Action	Signal State	Time Measured (s)	Distance Calculated (cm)	Output
1	Send trigger pulse (10µs HIGH)	Trigger HIGH
2	Wait for echo to go HIGH	Echo LOW to HIGH
3	Start timing echo pulse	Echo HIGH
4	Wait for echo to go LOW	Echo HIGH to LOW	0.015
5	Calculate duration		0.015
6	Calculate distance (duration * 34300 / 2)			257.25
7	Output distance			257.25	257.25 cm
8	End measurement cycle				Measurement complete

💡 Measurement ends after echo pulse duration is measured and distance is calculated.

Variable Tracker

Variable	Start	After Step 4	After Step 6	Final
trigger	LOW	HIGH (10µs pulse)	LOW	LOW
echo	LOW	HIGH (echo start)	LOW (echo end)	LOW
duration	0	0.015	0.015	0.015
distance	0	0	257.25	257.25

Key Moments - 3 Insights

Why do we wait for the echo signal to go HIGH and then LOW?

Because the time between echo HIGH and LOW is the pulse duration that tells us how long the sound took to return, as shown in steps 3 and 4 in the execution_table.

Why do we multiply duration by 34300 and then divide by 2 to get distance?

34300 cm/s is the speed of sound. We multiply by duration to get total travel distance and divide by 2 because the sound travels to the object and back, explained in step 6.

What happens if the echo signal never goes LOW?

The measurement would hang waiting for echo end, so the program must handle timeouts to avoid infinite wait, which is not shown here but important in real use.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'duration' after step 4?

A 0 seconds

B 0.015 seconds

C 10 microseconds

D 257.25 centimeters

💡 Hint

Check the 'Time Measured (s)' column at step 4 in the execution_table.

At which step is the distance actually calculated?

A Step 6

B Step 5

C Step 3

D Step 7

💡 Hint

Look at the 'Distance Calculated (cm)' column in the execution_table.

If the echo pulse duration doubles, how does the distance change?

A Distance stays the same

B Distance halves

C Distance doubles

D Distance quadruples

💡 Hint

Refer to the formula in step 6: distance is proportional to duration.

Concept Snapshot

DistanceSensor (ultrasonic) measures distance by:
- Sending a short trigger pulse
- Measuring echo pulse duration
- Calculating distance = (duration * speed_of_sound) / 2
- Outputting distance in cm
Use gpiozero library on Raspberry Pi for easy access.

Full Transcript

This visual trace shows how an ultrasonic DistanceSensor works on Raspberry Pi. First, the sensor sends a short trigger pulse. Then it waits for the echo signal to go HIGH, marking the start of the echo pulse. It times how long the echo stays HIGH until it goes LOW again. This duration is the time sound took to travel to the object and back. Using the speed of sound (34300 cm/s), the program calculates distance by multiplying duration by speed and dividing by two. Finally, it outputs the distance in centimeters. Variables like trigger and echo signals change state during the process. Key moments include understanding why we wait for echo HIGH and LOW, the distance formula, and handling echo timeouts. The quizzes test understanding of timing, calculation steps, and proportionality of distance to duration.