HC-SR04 Datasheet: Analysis

The HC-SR04 is an ultrasonic sensor detector that can measure distances from 2 cm (0.787 in) to 400 cm (13.123 ft). Unlike infrared sensors, sunlight or black material does not inhibit performance (acoustically absorbent materials – e.g., cloths – can complicate detection). A breakout board for the sensor comes with an ultrasonic transceiver module, and the 2.54 mm header pins make for easy interfacing with a breadboard for microcontroller prototyping. The breakout board has four pins (from L to R when viewing from the board top):

HC-SR04 Breakout Board Pin Descriptions

• VCC: +5 V DC source
• Trig: Trigger input of the sensor
• Echo: Echo output of the sensor
• GND: Reference/return

To start the measurement, Trig must receive a pulsed logic-high signal (5V) for at least 10 us; after pulsing at the appropriate voltage and duration, the sensor will output eight cycles of ultrasonic burst at 40 kHz and wait for the reflected ultrasonic burst. When the sensor detects an ultrasonic burst from the receiver, it sets the Echo pin high and delays the distance from the reflection proportionally. By measuring the pulse width duration, 

Time = Echo pulse width (in us)

the distance to the reflected surface can be easily determined.

• Distance in centimeters = Time / 58
• Distance in inches = Time / 148

Connect the ground pin before VCC to prevent damage to the HC-SR04 device. The detection surface should be 0.5 m2 or greater in area for best results. 

Applications for an ultrasonic sensor detector include:

• Car distance sensors – While backup cameras tend to dominate modern vehicles, distance sensors can act as a “last line of defense” by alerting drivers when they’re 0.3 m (approximately 1 ft.) to the closest surface. 
• Liquid level detection – For greater accuracy, the HC-SR04 positioned above a pipe perpendicular to a liquid reservoir can prevent unstable readings due to surface waves, bubbles, etc.

HC-SR04 Code Implementation

Programming for the HC-SR04 is straightforward, using any microcontroller with a digital readout. The code to manipulate the HC-SR04 will differ according to the language, but developers should consider the basic structure of the code blocks:

• Declarations – Set up variables for the Trig and Echo pins; these data types must be writable at high and low logic levels.
• Initialization – Start serial communication and set Trig logic low.
• Loop – Trig will alternate between logic high and low levels, sandwiching a short delay. The trigger oscillation will continue while the sensor determines the distance to the incident surface.
• Conditional – The console prints an error message if the duration (measured as a timing delay) exceeds the sensor’s maximum operating distance. If the duration is within the sensor’s operating distance, it converts the value to the desired units and prints it to the console.

Ultra Librarian Accelerates Product Development

The HC-SR04 datasheet provides the basic functionality and circuit characteristics necessary to create an effective design with the device for ultrasonic sensing. The advantage of the HC-SR04 (and other similar sensor breakout boards) is the ability to prototype rapidly with breadboards (and more customized prototype boards). When time is of the essence during prototyping revisions, Ultra Librarian’s verified library containing millions of component footprints, schematic symbols, and simulation models can free up workflows while maintaining manufacturability. Best of all, getting started with Ultra Librarian is free.

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