说明：  介绍时基电路555（556）工作原理，分大类介绍IC555在波形产生、变换、调制、定时、计量、检测、电源变换、继电保护等多方面的应用技术。
(This paper introduces the working principle of time base circuit 555 (556), and introduces the application technology of IC555 in waveform generation, transformation, modulation, timing, measurement, detection, power transformation, relay protection, etc.)