Introduction to photoelectric sensors

Introduction to photoelectric sensors

Photoelectric sensor is a kind of sensor that converts optical signal into electrical signal. It has the advantages of simple structure, high precision, fast response speed, non-contact, etc. It is widely used in detection and control systems.

The physical basis of photoelectric sensors is the photoelectric effect. This phenomenon is that when light irradiates an object, the object is bombarded by a series of energy photons, and the electrons in the object material absorb the photon energy and respond to electrical effects, such as changes in conductivity, emission of electrons, or generation of electromotive force. The photoelectric effect is divided into external photoelectric effect and internal photoelectric effect. The internal photoelectric effect is further divided into the photoconductive effect and the photovoltaic effect. Photoelectric elements based on the photovoltaic effect include photovoltaic cells, phototransistors, and the like. The following describes the photoelectric sensors commonly used in solar photovoltaic power generation tracking devices – photoelectric cells, photodiodes, phototransistors and PSD photoelectric position sensors.

1. Micro photocell type

The photoelectromotive force and photocurrent of the photocell are different under different illumination. Generally speaking, the relationship between open circuit voltage and illuminance is nonlinear, while the short-circuit current has a linear relationship with illuminance in a large range. The smaller the load resistance, the better the linear relationship, and the wider the linear range. Therefore, when detecting continuously changing illuminance, the load resistance should be reduced as much as possible, so that the photovoltaic cell works in a state close to a short circuit, that is, the photovoltaic cell is used as a current source. When the optical signal changes intermittently, the photovoltaic cell can also be used as a voltage source. The advantage is that the adjustment is more precise and the circuit is relatively simple.

2. Photodiode

The photodiode CDS is a photoresistor whose resistance value changes with the change of light intensity. As a sensor to detect changes in sky light and track the position of the sun, the characteristics of the photoresistor are the closest to the human eye, so it is suitable for the measurement of visible light. The CDS with model GL5516 has a dark resistance of 100kΩ and a bright resistance of 5~10kΩ. The relationship between the resistance change of CDS and the change of light is linear, and the change of its resistance value rises or falls along a straight line within a range.

3. Phototransistor

There are two PN junctions inside the phototransistor, PNP type and NPN type. Different from the general triode, its emitter side is small in size to expand the light area. It can be regarded as a combination of a photodiode and a transistor equivalently. When the base is open, the base-collector is in reverse bias, and the photocurrent Ici formed when there is light is amplified by the transistor as the base current. The amplification principle is the same as that of the general transistor, so that Ici is amplified by β times, and the general magnification β is several tens of times, so the sensitivity of the phototransistor is dozens of times higher than that of the photodiode. Compared with photodiodes, phototransistors have larger dark current, larger noise, and larger junction capacitance, thereby increasing the response time. The commonly used materials for phototransistors are silicon and wire. The dark current of silicon phototransistor is very small. Generally, there is no external lead of the base, but only two leads of the emitter and the collector. The phototransistor also has a base electrode, which is generally used for temperature compensation. The dark current of the wrong phototransistor is large. In order to increase the ratio of photocurrent to dark current, a resistor is often connected between the emitter and the base.

Generally speaking, the photoelectric characteristics of the photodiode are linear when the illumination is small, while the photocurrent of the phototransistor increases less with the illumination when the illumination is small, and when the illumination is large enough, the output current is saturated. This is because the current magnification of the phototransistor decreases at low current and high current. The advantage is that the adjustment accuracy is high, but the implementation circuit is too complicated. The photocurrent of the photodiode is small, the output characteristic linearity is good, and the response time is fast; the phototransistor’s photocurrent is large, the output characteristic linearity is poor, and the response time is slow. Generally, a switching circuit with high sensitivity and low operating frequency is required to use a phototransistor, and a photodiode should be used when a linear relationship between the photocurrent and the illuminance is required or when it is required to work at a high frequency. Since the phototransistor and diode are made of silicon-based materials, it is a broad-spectrum light sensor, and its response peak is 850nm, which is extremely susceptible to near-infrared and ultraviolet interference, which makes the stability of the whole machine made of it poor. Regardless of photodiode or phototransistor, they are not only sensitive to infrared rays, but also have a role in strong sunlight and light. When the light is too strong, the output of the amplifier circuit will be saturated and out of control, and red plexiglass should be added to filter to reduce the impact of ambient light.

The light intensity detection range of the phototransistor is wide, but its linearity is poor. The photocell has the widest light intensity detection range, good product consistency, and very good linearity of its output current within a wide range of light intensity conversion, so it is very suitable as a feedback light intensity detection element.

4. PSD photoelectric position sensor

Semiconductor photoelectric position sensor PSD (Position Sensitive Device) is a photoelectric position sensitive detector based on the lateral photoelectric effect, which can convert the position of the light spot on the photosensitive surface into an electrical signal. Using the lateral photoelectric effect on the PN junction of the PSD can detect the irradiation position of the incident light spot, which can be directly used to measure the position, distance, height, angle and motion trajectory, etc.;

The PSD device can detect the position of the light spot without dead zone, and has the characteristics of high sensitivity, good repeatability, high resolution, fast response speed and simple circuit configuration. Due to its very high measurement accuracy, the accuracy of the measurement angle can be easily achieved within 0.1°. In order to cooperate with the solar panel tracking device to achieve its precision, the mechanical transmission structure will be extremely complicated, and the solar panel without a concentrating device is not very sensitive to the angle of light intensity. So it should be considered on concentrating solar panels, but not necessary on non-concentrating solar panels.