Photoelectric sensors work by controlling the change in light intensity into a change in electrical signal. A photoelectric switch is a small electronic device that detects changes in the received light intensity. Photoelectric sensors are usually composed of three parts: transmitter, receiver and detection circuit.
The transmitter has a calibrated lens that focuses the light toward the receiver, and the receiver cable connects the unit to a vacuum tube amplifier. There is a small incandescent lamp in the metal cylinder as the light source. These small and sturdy incandescent lamp sensors are the prototype of today's photoelectric sensors.
The transmitter is aimed at the target emitting beam, and the emitted beam is generally derived from a semiconductor light source, a light emitting diode (LED), a laser diode, and an infrared emitting diode. The beam is emitted uninterrupted or the pulse width is varied.
The receiver is composed of a photodiode, a phototransistor and a photocell. In front of the receiver, optical components such as lenses and apertures are mounted. Behind it is a detection circuit that filters out the valid signal and applies the signal. Photodiodes are now the most common sensor. Photoelectric sensor photodiode has the same appearance as a general diode, except that its tube has a window embedded with glass to facilitate light injection. In order to increase the light receiving area, the area of ​​the PN junction is made larger, and the photodiode works. In the reverse biased operating state, and in series with the load resistance, when there is no light, it is the same as the ordinary diode, the reverse current is very small, called the dark current of the photodiode; when there is light, the carrier is Excitation generates electron-holes called photocarriers.
Under the action of an external electric field, the photocarriers participate in conduction, forming a reverse current much larger than the dark current, which is called photocurrent. The magnitude of the photocurrent is proportional to the intensity of the light, so that an electrical signal that varies with changes in illumination intensity is obtained at the load resistance.
The photoelectric switch phototransistor has the function of converting the optical signal into an electric signal in addition to the photodiode, and also has the function of amplifying the electric signal. The appearance of the photosensitive three-stage tube is not much different from that of the general triode. Generally, the phototransistor only draws two poles - the emitter and the collector, the base is not led out, and the shell also opens the window for light to enter. In order to increase the illumination, the area of ​​the base area is made large, the emission area is small, and the incident light is mainly absorbed by the base area. When working, the collector junction is reversed and the emitter junction is forward biased. The current flowing through the tube when there is no light is dark current Iceo=(1+β)Icbo, which is smaller than the penetration current of the general triode; when there is illumination, a large number of electron-hole pairs are excited, so that the base generates The current Ib increases, and the current flowing through the tube is called photocurrent, and the collector current Ic=(1+β)Ib. The visible light sensor phototransistor has higher sensitivity than the photodiode.
In addition, the structural elements of the photosensor include a launching plate and an optical fiber. The corner reflector is a structurally sound emitting device that consists of a small triangular pyramidal reflective material that allows the beam to be accurately returned from the reflector. It can change the angle of emission from the range of 0 to 25 with the optical axis, so that the beam is almost returned from a line after it has been reflected.
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