Buzzer.

　　Buzzer is an electronic buzzer with integrated structure, which is powered by DC voltage. It is widely used as a sounding device in computers, printers, copiers, alarms, electronic toys, automotive electronic equipment, telephones, timers and other electronic products. Buzzers are mainly divided into piezoelectric buzzers and electromagnetic buzzers.

　　The buzzer is represented by the letter "H" or "HA" in the circuit (the old standard is "FM", "ZZG", "LB", "JD", etc.).

　　That's a special time to cross the line of fire. Generally speaking, those who can cross the wall and enter some places that people can't see. Or send a specific circuit to have a look.