The HC-SR04 is a cheap ultrasonic sensor. It is used to measure the range between itself and an object. Each HC-SR04 module includes an ultrasonic transmitter, a receiver, and a control circuit. You can use this sensor to detect human presence in a range of 2 cm to 5 cm, depending on sensor accuracy.

In general, the HC-SR04 module has four pins: GND, VCC, Trigger, and Echo. You can see the HC-SR04 from SparkFun (https://www.sparkfun.com/products/13959) in the following image. You can also find this sensor on SeeedStudio: https://www.seeedstudio.com/Ultra-Sonic-range-measurement-module-p-626.html. To save costs, you can buy the HC-SR04 sensor from Aliexpress. To learn how to use this sensor, refer to Chapter 2, A Smart Parking System.