I. What are the classifications of photoelectric switches?
① Diffuse reflection photoelectric switch: This 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, thus generating 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-reflective photoelectric switch: It also integrates the transmitter and receiver. The light emitted by the photoelectric switch transmitter is reflected back to the receiver by the reflector. When the object being detected passes by and completely blocks the light, the photoelectric switch generates a detection switch signal.
③ Through-beam photoelectric switch: It consists of a transmitter and a receiver that are structurally separated and positioned opposite each other on their optical axes. The light emitted by the transmitter directly enters the receiver. When the object being detected passes between the transmitter and receiver and blocks the light, the photoelectric switch generates a switching signal. When the object being detected is opaque, the through-beam photoelectric switch is the most reliable detection device.
④ Slot-type photoelectric switch: It typically adopts a standard U-shaped structure, with its transmitter and receiver located on opposite 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. Slot-type photoelectric switches are well-suited for detecting high-speed moving objects, and they can distinguish between transparent and translucent objects, making them safe and reliable to use.
⑤ Fiber optic photoelectric switch: It uses a plastic or glass fiber optic sensor to guide light, enabling detection of objects at a distance. Fiber optic sensors are typically divided into through-beam and diffuse reflection types.
II. Differences between NPN and PNP photoelectric switches
The main difference between NPN and PNP photoelectric switches lies in the type of transistor. The transistor is a key component in a photoelectric switch, used to convert optical signals into electrical signals. The different conduction types and connection methods of NPN and PNP transistors lead to differences in their operating principles and application scenarios.
2.1 NPN type photoelectric switch
The NPN type photoelectric switch uses an NPN type transistor as its core component. An NPN type transistor consists of N-type and P-type semiconductors and has three pins: collector (C), emitter (E), and base (B). When a positive voltage is applied between the base and emitter, the transistor conducts, forming a conductive path between the collector and emitter, allowing current to flow from the collector to the emitter.
2.2 PNP type photoelectric switch
PNP photoelectric switches use PNP transistors as their core components. A PNP transistor consists of P-type and N-type semiconductors and has three pins: collector (C), emitter (E), and base (B). Unlike NPN transistors, PNP transistors are cut off when a forward voltage is applied between their base and emitter, and no conductive path is formed between the collector and emitter. The transistor only conducts when a reverse voltage is applied between its base and emitter.
III. How to distinguish between NPN and PNP photoelectric switches
There are many ways to distinguish between NPN and PNP photoelectric switches. Here are some commonly used methods:
3.1 Consult the product manual or datasheet.
Product manuals or datasheets usually indicate the type of photoelectric switch. When purchasing a photoelectric switch, you can request a product manual or datasheet from the supplier to check the type of photoelectric switch.
3.2 Observe the pin arrangement
The pin arrangements of NPN and PNP photoelectric switches are usually different. The pin arrangement of an NPN photoelectric switch is: emitter (E), base (B), and collector (C), while the pin arrangement of a PNP photoelectric switch is: collector (C), base (B), and emitter (E). By observing the pin arrangement, one can initially determine the type of photoelectric switch.
3.3 Measurement using a multimeter
A multimeter can be used to measure the on and off states of a photoelectric switch, thus determining its type. The following are the steps for measuring NPN and PNP type photoelectric switches using a multimeter:
3.3.1 Measurement of NPN type photoelectric switch
a) Set the multimeter to the diode measurement setting.
b) Connect the red probe of the multimeter to the emitter (E) of the photoelectric switch and the black probe to the collector (C).
c) Observe the multimeter reading. If the reading is around 0.6V (the forward voltage drop of the silicon diode), it indicates that the photoelectric switch is in the conducting state.
d) Connect the red probe of the multimeter to the collector (C) of the photoelectric switch and the black probe to the emitter (E).
e) Observe the multimeter reading. If the reading is infinity (OL), it indicates that the photoelectric switch is in the off state.
