I. Introduction Photoelectric switches are members of the sensor family. They convert changes in light intensity between the transmitter and receiver into changes in current to achieve detection. Because the output and input circuits of a photoelectric switch are electrically isolated (i.e., electrically insulated), it can be used in many applications. II. Introduction to Photoelectric Switches 1. Working Principle A photoelectric switch (photoelectric sensor) is short for photoelectric proximity switch. It utilizes the blocking or reflection of a light beam by the object being detected. A synchronous circuit selects the circuit, thereby detecting the presence or absence of the object. The object is not limited to metal; any object that can reflect light can be detected. The photoelectric switch converts the input current into a light signal at the transmitter, and the receiver then detects the target object based on the intensity or presence/absence of the received light. The working principle is shown in Figure 1. Most photoelectric switches use infrared light with wavelengths close to visible light. Figure 2 shows some of the photoelectric switches from the German company SICK. 2. Classification and Terminology of Photoelectric Switches (1) Classification ① Diffuse Reflection Photoelectric Switch: It is a sensor that integrates a transmitter and a receiver. When a detected object passes by, the object reflects a sufficient amount of light emitted by the photoelectric switch transmitter to the receiver, and the photoelectric switch generates a switching signal. When the surface of the detected object is bright or its reflectivity is extremely high, the diffuse reflection photoelectric switch is the preferred detection mode. ② Mirror Reflection Photoelectric Switch: It also integrates a transmitter and a receiver. The light emitted by the photoelectric switch transmitter is reflected back to the receiver by a mirror. When the detected object passes by and completely blocks the light, the photoelectric switch generates a detection switching signal. ③ Through-beam Photoelectric Switch: It includes a transmitter and a receiver that are structurally separated and placed opposite each other on their optical axes. The light emitted by the transmitter directly enters the receiver. When the detected object passes between the transmitter and the receiver and blocks the light, the photoelectric switch generates a switching signal. When the detected object is opaque, the through-beam photoelectric switch is the most reliable detection device. ④ Slot-type photoelectric switch: It usually adopts a standard U-shaped structure, with its transmitter and receiver located on both sides of the U-shaped slot, forming an optical axis. When the object being detected passes through the U-shaped slot and blocks the optical axis, the photoelectric switch generates a switching signal. The slot-type photoelectric switch is more suitable for detecting objects moving at high speeds, and it can distinguish between transparent and semi-transparent objects, making it safe and reliable to use. ⑤ Fiber optic photoelectric switch: It uses plastic or glass fiber optic sensors to guide the light, and can detect objects at a distance. Fiber optic sensors are usually divided into through-beam and diffuse reflection types. Their working light diagrams are shown in Figure 3. (2) Explanation of terms Common terms are shown in Figure 4. ① Detection distance: refers to the spatial distance from the reference position (sensing surface of the photoelectric switch) measured when the switch is activated, when the detection body moves in a certain way. Rated operating distance refers to the nominal value of the near-switch operating distance. ② Hysteresis distance: the absolute value between the operating distance and the reset distance. ③ Response frequency: the number of times the photoelectric switch is allowed to cycle within a specified 1s time interval. ④ Output state: divided into normally open and normally closed. When no object is detected, the load connected to the normally open photoelectric switch does not work because the output transistor inside the photoelectric switch is cut off. When an object is detected, the transistor conducts, and the load is powered on. ⑤ Detection method: According to the different paths by which the light emitted by the transmitter is reflected back to the receiver when the photoelectric switch detects an object, it can be divided into diffuse reflection, specular reflection, and through-beam type. ⑥ Output form: There are several commonly used output forms, including NPN two-wire, NPN three-wire, NPN four-wire, PNP two-wire, PNP three-wire, PNP four-wire, AC two-wire, AC five-wire (with built-in relay), and DC NPN/PNP/normally open/normally closed multi-function. ⑦ Pointing angle: See the schematic diagram of the pointing angle of the photoelectric switch, as shown in the three small diagrams at the bottom of Figure 4. ⑧ Surface reflectivity: The light emitted by the diffuse reflection photoelectric switch needs to pass through the surface of the detected object before it is reflected back to the receiver of the diffuse reflection switch. Therefore, the detection distance and the surface reflectivity of the detected object will determine the intensity of the light received by the receiver. The intensity of light reflected back from a rough surface will be less than that reflected back from a smooth surface. Moreover, the surface of the object being detected must be perpendicular to the light emitted by the photoelectric switch. The reflectivity of commonly used materials is shown in Table 1. [align=center] [/align] ⑨ Environmental characteristics: The environment in which the photoelectric switch is used will also affect its long-term working reliability. When the photoelectric switch is working at its maximum detection distance, the optical lens will be stuck to dirt in the environment, or even corroded by some strong acidic substances, which will reduce its operating parameters and reliability. A simpler solution is to determine the optimal working distance by derating the photoelectric switch according to its maximum detection distance (Sn). (3) Precautions for use ① The infrared sensor is a diffuse reflection type product, and the standard detection body used is a flat white drawing paper. ② The infrared photoelectric switch can work stably under high ambient light conditions, but in principle, the sensor optical axis should be avoided from being directly facing strong light sources such as sunlight. ③ The minimum detectable width of the through-beam photoelectric switch is 80% of the width of the lens of this type of photoelectric switch. ④ When using inductive loads (such as lamps, motors, etc.), the transient inrush current is large, which may degrade or damage the AC two-wire photoelectric switch. In this case, please use an AC relay to switch the load. ⑤ The lens of the infrared photoelectric switch can be wiped with lens paper. Do not use diluents or other chemicals to avoid permanent damage to the plastic lens. ⑥ To meet the actual requirements of users in some harsh conditions, such as dusty environments, the sensitivity of the photoelectric switches produced has been increased by 50% to meet the requirements of extending the maintenance cycle of the photoelectric switch during long-term use. ⑦ All products are manufactured using SMD technology and have passed strict testing before leaving the factory. Under normal use, they will not be damaged. To avoid accidents, please check whether the wiring is correct and whether the rated voltage is the rated value before connecting the power supply. See Figure 5 for a schematic diagram of the above precautions. III. Application Examples Figure 6 shows the various applications of photoelectric switches. Figure 6(a) illustrates the use of a photoelectric switch for material positioning and shearing control; Figure 6(b) shows the use of a photoelectric switch to control the upper and lower limits of the liquid level. When the liquid level is higher or lower than the upper or lower limit, the photoelectric switch control circuit can open or close the threshold gate to keep the liquid level between the upper and lower limits; Figure 6(c) utilizes the light-blocking effect of an object to detect the number of objects passing through or the presence of an object; Figure 6(d) utilizes the rectilinear propagation of light to check whether products are arranged at the same height; Figure 6(e) shows the use of a photoelectric switch on an assembly line to detect the number of products; and Figure 6(f) shows the use of a photoelectric switch to detect the liquid level. In conclusion, besides the examples mentioned above, photoelectric switches have been applied in many other areas, such as stroke control, diameter limiting, speed detection, and air flow control. We believe that photoelectric switches will become increasingly advanced, and their applications will become more widespread. References [1] Yang Chongzhi. Handbook of Special New Electronic Components [M]. Shenyang: Liaoning Science and Technology Press, 2001 [2] Li Kejie. New Handbook of Sensor Technology [M]. Beijing: National Defense Industry Press, 2003