Because displacement sensors can be made according to different principles and methods, and different displacement sensors have different methods of perceiving objects, common proximity switches are as follows:
1. Inductive proximity switch
This type of switch is sometimes called an eddy current proximity switch. It utilizes a conductive object to generate eddy currents inside the object when approaching a proximity switch that can generate an electromagnetic field. This eddy current reacts on the proximity switch, causing changes in the internal circuit parameters of the switch, thereby identifying whether a conductive object is moving closer and controlling the on or off of the switch. The object that can be detected by this proximity switch must be a metal object, which is generally widely used in logistics, automated production lines, and mechanical equipment. 
2. Photoelectric proximity switch
A switch made using the photoelectric effect is called a photoelectric switch. Install the light-emitting device and photoelectric device in the same detection head in a certain direction. When a reflective surface (object being detected) approaches, the photoelectric device receives the reflected light and outputs a signal, thereby "perceiving" an object approaching.
3. Capacitive proximity switch
The measurement of this type of switch is usually based on one electrode that makes up the capacitor, and the other electrode is the outer shell of the switch. This casing is usually grounded or connected to the equipment casing during the measurement process. When an object moves towards a proximity switch, regardless of whether it is a conductor or not, its proximity always causes a change in the dielectric constant of the capacitor, resulting in a change in capacitance and a change in the state of the circuit connected to the measuring head. This can control the connection or disconnection of the switch. The objects detected by this proximity switch are not limited to conductors, liquids or powders that can be insulated. 
4. Hall proximity switch
Hall element is a type of magnetic sensing element. A switch made using Hall elements is called a Hall switch. When a magnetic object moves close to a Hall switch, the Hall element on the detection surface of the switch produces a Hall effect, causing a change in the internal circuit state of the switch, thereby identifying the presence of a nearby magnetic object and controlling the on or off of the switch. The detection object of this proximity switch must be a magnetic object, which is commonly used in black and white household appliances. 
5. Passive proximity switch
This type of switch does not require a power source and controls the closed state of the switch through magnetic induction. When a magnetic or iron trigger approaches the switch magnetic field, it is controlled to close by the internal magnetic force of the switch. Features: No power required, non-contact, maintenance free, environmentally friendly. 
6. Pyroelectric proximity switch
A switch made of components that can sense temperature changes is called a pyroelectric proximity switch. This type of switch installs a pyroelectric device on the detection surface of the switch. When an object with a different ambient temperature approaches, the output of the pyroelectric device changes, thus detecting an object approaching. 
7. Other types of proximity switches
When the observer or system changes the distance to the wave source, the frequency of the approaching wave will shift, and this phenomenon is called the Doppler effect. Sonar and radar are made using the principle of this effect. Doppler effect can be used to make ultrasonic proximity switches, microwave proximity switches, etc. When an object approaches, the reflected signal received by the proximity switch will generate a Doppler frequency shift, which can identify whether an object is approaching.