555 定时器是一种模拟和数字功能相结合的中规模集成器件。一般用双极性工艺制作的称为 555，用 CMOS 工艺制作的称为 7555，除单定时器外，还有对应的双定时器 556/7556。555 定时器的电源电压范围宽，可在 4.5V~16V 工作，7555 可在 3~18V 工作，输出驱动电流约为 200mA，因而其输出可与 TTL、CMOS 或者模拟电路电平兼容。(The 555 timer is a medium scale integrated device that combines analog and digital functions. In general, the two-polarity process is made of 555, and the CMOS process is called 7555. Besides the single timer, there is also a corresponding two-timer 5556/7556. The power voltage range of 555 timer is wide, which can work at 4.5v ~ 16V, 7555 can work in 3~ 18V, the output drive current is about 200mA, so its output can be compatible with TTL, CMOS or analog circuit level.)