The HC-SR04 is a ultrasonic distance measuring sensor module that uses ultrasonic sound waves to measure the distance to an object. It consists of a transmitter and a receiver that are housed in a single package. To measure the distance to an object, the HC-SR04 sends out a short ultrasonic pulse and measures the time it takes for the pulse to bounce back after it hits the object. The distance to the object can then be calculated based on the speed of sound and the time it took for the pulse to return. The HC-SR04 is a low-cost, easy-to-use distance measuring sensor that is widely used in a variety of applications, such as robotics, industrial automation, and home automation. It is typically interfaced with a microcontroller or other control device, which can read the distance measurements and use them to control various devices or systems.

The HC-SR04 Ultrasonic ranging module will allow you to detect whether there are obstacles in the way at a specified distance.

Simply supply the module with 5V and the output will be 5V while an obstacle is in range, or 0V if not in range. Specific ranges can be detected using specific timings. 

• Module Working Principle:
• 1.Adopt IO trigger through supplying at least 10us sequence of high level signals.
• 2.The module will automatically send eight 40khz square wave and automatically detect whether it received
• the returning pulse signal.
• 3.If there is signal returning, the output will be HIGH,
• Test distance = (high level time * sound velocity (340M/S) / 2.

An instructable tutorial on how to use the HC-SR04 module with an Arduino can be found at: https://howtomechatronics.com/tutorials/arduino/ultrasonic-sensor-hc-sr04/