The HC-SR04 Ultrasonic range sensing module is great for sensing distances that can be used as an input for your Arduino projects, but using multiple (more than two) can quickly cost you a lot of pins.
This basic Instructable gives you the code and basic instructions to use the 74HC595 8 bit series to parallel shift register so you can connect up to 8 HC-SR04 modules (16 IO pins) while using only 4 Arduino IO pins
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Step 1: Arduino Code
^^Basic layout of shift register IC and picture with only one HC-SR04 sensor^^
The attached File contains the needed Arduino code and pin layout (scanping)
Also attached is basic code for using the HC-SR04 Ultrasonic range sensing module (ping)