Ultrasonic Sensor: Construction and Working Principle

This page describes the construction and working principle of an ultrasonic sensor, often used as a distance sensor.

Definition: An ultrasonic sensor is a transducer that converts electrical energy into sound waves, and vice versa. These sound waves operate above the normal range of human hearing, hence the term “ultrasonic.” These waves have frequencies above approximately 18,000 Hz.

Ultrasonic Sensor Construction and Working

Here’s a look at the components and functionality:

alt: Ultrasonic Sensor

• The ultrasonic sensor has two main parts: a transmitter and a receiver.
• The transmitter converts electrical energy into sound waves and emits them.
• The receiver captures the echo (reflected sound waves) and converts it back into electrical energy.
• The sensor measures the time it takes for the echo to return, which is then used to calculate the distance to the object.

Essentially, the sensor calculates the time interval between signal transmission and echo reception, determining the object’s distance. Because of this function, it’s often called a distance sensor.

• Piezoelectric crystals are commonly used in ultrasonic sensor construction because they can oscillate at high frequencies.

alt: ultrasonic sensor pin diagram

Figure 2 shows a typical pin diagram for an ultrasonic sensor. It usually has four pins:

• +5V (Vcc): Power supply pin.
• GND: Ground pin.
• Trigger: Input pin to initiate a measurement.
• Echo: Output pin that signals the duration of the echo return.

Ultrasonic Sensor Working Principle: Step-by-Step

alt: Ultrasonic sensor signals

Let’s break down how an ultrasonic sensor works:

• STEP-1: Set the ‘TRIGGER’ pin high for a short period (e.g., 10µs). This starts the sensor cycle.
• STEP-2: The transmitter emits eight pulses at a frequency of 40 KHz. After this transmission, the ‘ECHO’ pin transitions from low to high.
• STEP-3: The transmitted signal bounces off any nearby object and returns to the sensor as an echo.
• STEP-4: When the sensor detects the reflected wave, the ‘ECHO’ pin goes low.
• STEP-5: The duration for which the ‘ECHO’ pin remains high represents the time it took for the signal to travel to the object and back. This time is directly proportional to the distance.
• STEP-6: If no object is detected, the ‘ECHO’ pin typically remains high for a maximum duration (e.g., 38ms) before returning to a low state. This indicates that the sensor didn’t receive a valid echo within its range.

Technical Specifications

Typical specifications for an ultrasonic sensor include:

• Nominal Frequency output: 40 KHz
• Coverage range: 0.2 to 6 meters
• Receiver sensitivity: -67 dBm
• Sound pressure level: 112 dB (minimum)
• Maximum voltage input: 20 V(rms)

Applications of Ultrasonic Sensors

Ultrasonic sensors are used in various applications, including:

• Automatic car parking systems
• Humanoid robot design