Principle of photoelectric sensor

There are various optical measurement and control systems made based on the principle of the effect of light flux on photoelectric elements. According to the output properties of photoelectric elements (optical measurement and control systems), they can be divided into two categories: analog photoelectric sensors and pulse (switch) photoelectric sensors. Analog photoelectric sensors convert the measured light into a continuously changing photocurrent, which has a single value relationship with the measured light Analog photoelectric sensors can be divided into three categories based on the method of being measured (detecting target objects): transmission (absorption), diffuse reflection, and shading (beam blocking). The so-called transmission type refers to the situation where the measured object is placed in the optical path, and the light energy emitted by a constant light source passes through the measured object, partially absorbed, and the transmitted light is projected onto the photoelectric element; The so-called diffuse reflection type refers to the light emitted by a constant light source projected onto the object being measured, and then reflected from the surface of the object being measured and projected onto the photoelectric element; The so-called shading type refers to the situation where a portion of the luminous flux emitted by the light source is blocked by the measured object, causing a change in the luminous flux projected onto the photoelectric element. The degree of change is related to the position of the measured object in the optical path.

Photodiodes are the most common type of light sensor. The appearance of a photodiode is the same as that of a regular diode. When there is no light, it has a small reverse current, which is called the dark current of a photodiode; When illuminated, charge carriers are excited, producing electron hole pairs, known as photocarriers. Under the action of an external electric field, photo carriers participate in conduction, forming a much larger reverse current than dark current, which is called photocurrent. The magnitude of photocurrent is directly proportional to the intensity of light, so an electrical signal that varies with the intensity of light can be obtained on the load resistor.

In addition to the function of converting light signals into electrical signals through photodiodes, phototransistors also have the function of amplifying electrical signals. The appearance of a photosensitive transistor is not much different from that of a general transistor. Generally, a photosensitive transistor only has two poles - the emitter and collector - and the base is not led out. The tube shell is also opened with a window for light to enter. To increase the illumination, the base area is made large, the emission area is small, and the incident light is mainly absorbed by the base area. During operation, the collector junction is reverse biased and the emitter junction is forward biased. The current flowing through the transistor in the absence of light is the dark current Iceo=(1+β) Icbo (very small), which is smaller than the penetration current of a typical transistor; When illuminated, a large number of electron hole pairs are excited, causing an increase in the current Ib generated at the base. The current flowing through the transistor at this moment is called photocurrent, and the emitter current Ie=(1+β) Ib. It can be seen that phototransistors have higher sensitivity than photodiodes.