Automatic control of electrical appliances in accordance with the signal or a certain amount of physical changes automatically. Automatic electrical controls for many types, here only introduce several commonly used electrical automatic control equipment such as contactless switch, current relay, speed relay, pressure type temperature relay and a transistor time relay.

Knife switch, button switch, relay, contactor and other electrical appliances is actually a switch, it can be in accordance with the requirements of the production process control to connect or disconnect the circuit, to achieve automatic control and automatic protection purposes. The advantages of this kind of contact with electrical switch is reliable 1 good mechanical strength, but the switching characteristics of stability, they have a series of disadvantages: such as slow movement speed, power consumption, low sensitivity, large volume and mechanical parts easy to wear.

The transistor not only has the function of amplification, but also has the switching characteristic. There are many outstanding advantages of no point switch, such as high speed, low power consumption, high sensitivity, small size, light weight and no mechanical wear.

In a transistor switching circuit, the control signals are electrical signals. The problem of switching circuit is to use the method and circuit to generate, transform, transmit, amplify and measure the signal. Therefore, the switching circuit is also called pulse circuit, which is widely used in the production process of automatic control and remote control, etc..