A proximity switch is also called the non-contact proximity switch, is the ideal electronic switch quantity sensor, when the metal detection approach switch induction area, switch to no contact, no pressure, no spark, electrical instruction, widely used in machine tools, metallurgy, chemical industry, textile, and printing industry, in the automatic control system can be used as a limit, counting, positioning control and automatic protection link, etc. A proximity switch is a kind of position switch without mechanical direct contact operation with moving parts. When the object is close to the induction surface of the switch to the action distance, the switch can be operated, to drive DC appliances or provide control instructions to the PLC device.

A proximity switch is a switch-type sensor (that is, contactless switch), it has the characteristics of a travel switch, or micro switch, and has sensing performance, reliable action, stable performance, fast frequency response, long application life, strong anti-interference ability, and has the characteristics of waterproof, corrosion resistance, etc. Products are inductance type, capacitance type, Hall, photoelectric type, pyroelectric type, and other forms.

1. Inductive proximity switch
An inductive proximity switch consists of three parts: an oscillator, a switch circuit, and an amplifier output circuit. The oscillator generates an alternating magnetic field. When the metal target is close to this magnetic field and reaches the induction distance, eddy currents are generated in the metal target, which leads to attenuation of the oscillation and even stop vibration. The changes in oscillator oscillation and vibration stop are processed by the later amplifier circuit and converted into switch signals, which trigger the drive and control devices, to achieve the purpose of non-contact detection.

（IM08 inductive proximity switches）

2. Capacitive proximity switch
This switch is usually measured made up of a plate of the capacitor, and the other plate is the switch shell, the shell in the process of measurement is usually connected to the chassis of the ground or with the device, when there are objects to move to close to switch, regardless of whether it is the conductor, due to its close, total want to make the capacitance of the dielectric constant change, Thus, the capacitance changes, so that the state of the circuit connected with the measurement head also changes, so that the switch can be switched on or off. The object detected by the close switch is not limited to the conductor, but also the liquid or powder that can be insulated.

3. Hall proximity switch
The Hall element is a kind of magnetic sensitive switch, the use of hall element to make the switch, called the hall switch, when magnetic objects nearer the hall switch, switch the detection of hall element for generating a circuit inside the hall effect and makes the switch state change, thus identifying the presence of magnetic objects, near, in turn, control switch on and off, the proximity switch to detect objects must be metallic objects.

（HG12 hall proximity switches）

4. Photoelectric proximity switch
The switch made of a photoelectric effect is called a photoelectric switch. The luminizer and photoelectric device are installed in the same detection head in a certain direction.
When a reflective surface (detected object) is close to it, the photoelectric device receives the reflected light and then the signal output, so that it can perceive it.

（PM18 photoelectric sensor)

5. Pyroelectric proximity switch
The switch made of the component that can sense the temperature change is called a pyroelectric proximity switch. This kind of switch is the pyroelectric part installed on the detection surface of the switch. When an object with a different ambient temperature is close to it, the output of the pyroelectric device will change, so that the object can be detected.
6. Other forms of the proximity switch

When the distance of the observer or system to the source of the wave changes, the frequency of the approaching wave will shift. This phenomenon is called the Puller effect. Sonar and radar are made of the principle of this effect, using the Doppler effect can be made of ultrasonic approach switch, microwave approach switch. When an object moves closer, the reflected signal received by the proximity switch will generate Doppler shift, which can identify whether there is an object approaching.