IMPORTANT: The sensor output signal (ECHO) on the HC-SR04 is rated at 5V. However, the input pin on the Raspberry Pi GPIO is rated at 3.3V. Sending a 5V signal into that unprotected 3.3V input port could damage your GPIO pins, which is something we want to avoid! We recommend you to use a small voltage divider circuit, to lower the sensor output voltage to something our Raspberry Pi can handle. For simplicity, we will skip this step.