Photoelectric sensors are small electronic devices and key components for achieving photoelectric conversion in various photoelectric detection systems. It is mainly a sensor that utilizes various properties of light to detect the presence or absence of objects and changes in surface state. Optoelectronic sensors have the characteristics of non-contact, fast response, and reliable performance, making them widely used in industrial automation devices and robots.

Photoelectric sensor
A photoelectric sensor generally consists of three parts: a light source, an optical path, and a photoelectric element. Convert the measured changes into changes in the optical signal, and then further convert the optical signal into an electrical signal using optoelectronic components.



Principle of photoelectric effect
Optoelectronic components are the most important component of photoelectric sensors, and their core working principle is different types of photoelectric effects. According to the wave particle duality, light is composed of photons moving at the speed of light. When an object is illuminated by light, its internal electrons absorb the energy of the photons and change their state, resulting in a change in their electrical properties. This phenomenon is called the photoelectric effect.

According to the different changes in electrical properties, the photoelectric effect can be divided into the following three types:
1) External photoelectric effect
The phenomenon of electrons escaping from the surface of an object under the action of light is called the external photoelectric effect. Optoelectronic elements based on external photoelectric effects include phototubes, photomultiplier tubes, etc


2) Photoconductive effect
The phenomenon where electrons in a semiconductor cannot jump out of the semiconductor after absorbing photons, causing a change in the conductivity of the object or generating a photo induced electromotive force, is called the internal photoelectric effect. The internal photoelectric effect can be divided into photoconductive effect and photogenerated volt effect according to its working principle. Optoelectronic components based on photoconductivity effects include photoresistors, phototransistors, etc



3) Photovoltage effect
The phenomenon of an object generating a certain directional electromotive force under the action of light is called the photogenerated volt effect. Optoelectronic components based on the photovoltaic effect include photocells, photodiodes, transistors, etc



Working principle of optoelectronic components
Based on different photoelectric effects, let's take a look at how they work:
External photoelectric effect device
Optoelectronic devices made by utilizing the external photoelectric effect of substances emitting electrons under light irradiation are generally vacuum or inflatable optoelectronic devices, such as photoelectric tubes and photomultiplier tubes.

Taking a photoelectric tube as an example, when the incident light is irradiated on the cathode, a single photon transfers all its energy to a free electron in the cathode material, thereby increasing the energy of the free electron. When the energy obtained by an electron is greater than the escape work of the cathode material, it can overcome the surface binding of the metal and escape, forming electron emission, which is called photoelectron. Photoelectrons are only generated when the frequency of incident light exceeds the limit frequency.



After the generation of photoelectrons, they are absorbed by the anode in the vacuum tube, thereby generating an electric current. If the light intensity is increased at this time, more photons will be irradiated onto the cathode material, resulting in more photoelectrons and a corresponding increase in photocurrent. When the resistance R value is determined, the photocurrent in the circuit is a function of the incident light intensity, thereby achieving photoelectric conversion. By reading the current through the measurement circuit, the light intensity can be calculated.