Photoelectric sensor, types and applications!

When we talk about the automation of homes and industrial processes, no doubt the sensors are present, as they are fundamental parts for automation. Due to the importance and wide variety of existing sensors, the World of Electrical will talk especially about photoelectric sensors, explaining what photoelectric sensors are , what are the types of photoelectric sensors, what are the applications of a photoelectric sensor and how does a photoelectric sensor work.

Photoelectric sensor

The photoelectric sensor was created with the purpose of converting a light signal into an electrical signal, so that this electrical signal can be processed with, for example, an electronic circuit. The photoelectric sensor is a type of sensor used for the most diverse applications, from security systems, control, industrial machines, medical equipment and embedded electronics.

The photoelectric sensor is a component that can be both a transducer and indeed a sensor. We can define the photoelectric sensor as a transducer because it is capable of converting light energy into electrical energy, as well as photocells, which directly convert light into electrical energy.

There are also sensors that convert light into a variation of any electrical quantity, that is, they can convert light into electrical resistance or voltage for example. We can mention LDRs and photodiodes as photoelectric sensors.

Knowing the huge amount of photoelectric sensors, for the most diverse purposes, how to define which sensor is ideal for each application? To help you, we separate the main types of photoelectric sensors and their main characteristics, then make the choice of the best photoelectric sensor according to your application. For a sensor that is not very sensitive in applications where light sources are weak, using a slow sensor in situations where it is necessary to detect the rapid movement of light or shadow can completely compromise a project, simply because of a poor choice. (this has to be the fourth paragraph of this part)

Sensor types: LDRs

LDRs (Light Dependent Resistors) or light-dependent resistors, are also known as photoresistors and are photoconductive type sensors. They are resistors that allow the passage of electric current according to the amount of light that falls on the base of the sensor. In an LDR, the greater the amount of light on its base, the lower its electrical resistance, thus allowing the passage of electrical current.

The LDR is a bidirectional sensor, which conducts electric current in both directions, that is, it can operate directly on alternating current circuits. In addition, the LDR is a slow device and is commonly used in automation applications, such as automatic lighting systems, traffic alarms, among others. It is not a suitable sensor for measuring instruments or for applications that require a high speed.

Photoelectric sensor LDR and its respective symbology.

Sensor types: Photodiodes

Photodiodes are electronic devices made of a semiconductor material, which has the property of varying their electrical resistance depending on the intensity of light that falls on them, that is, they work as a diode, but only if a certain amount of light falls on the photodiode.

Unlike LDRs, photodiodes are devices that have a high-speed characteristic, do not drive in both directions, only when fed in reverse, generally used for reading bar codes, rotation and position sensors, CD readers, DVD and Blu-ray, plus many other applications.

Photoelectric sensor (photodiode) and its respective symbology.

Sensor types: Phototransistors

The phototransistor has the principle of operation similar to the photodiode, which is the conduction of electric current when light falls on the phototransistor. They can be used in such a way that the current generated in the release of charges by the light is amplified, presenting again.

Photoelectric sensor (phototransistor) and its respective symbology.

Photoelectric sensor: Features

To make the right choice of the photoelectric sensor according to each application, we need to take into account the characteristics of the photoelectric sensor, such as its sensitivity, speed, and spectral response.


The sensitivity of a photoelectric sensor associates the magnitude of the output that varies according to the intensity of light that affects that sensor, that is, it is the ratio of the magnitude output to the input magnitude.


The response speed of a photoelectric sensor is essential in certain applications, as the sensors must detect very rapid variations in light, such as speed control of machines, barcode reading, among many other applications.

Spectral Response

The vast majority of photoelectric sensors are capable of detecting both infrared and ultraviolet radiation, that is, photoelectric sensors are able to “see” much more than human eyes.

If you want to learn more about photoelectric sensors, below we have a video from the World of Electrical that demonstrates step by step how to do tests on a photocell using a multimeter.v