Solid state relays (SSRs) have plenty of types and models, so its input control methods and control circuits are numerous accordingly. The common characteristics of solid state relay (SSR) lie in the small drive current or drive voltage, namely, the on-off the SSR can be controlled by just inputting a small signal. If it is required to realize effective and reliable control, its input signal should reach a given value. As to the requirement to the working current of the input loop, it is recommended to switch on when the value is 5-10mA and switch off when the value is lower than 1mA. As to the requirement of the working voltage of the input terminal, it is generally no lower than 3V in switch on, and lower than 1V in switch off. Of course, because of the different product characteristics and different manufacturers, the requirement to the input current or resistance value is also different. Therefore, in using and designing the circuit, the selection and design can be conducted according to the specification and operation occasions on the basis of solid state relay (SSR) type. The following text introduces some basic methods and circuits of SSR input control.