Photosensitive Sensor - Photosensitive Sensor Features - Photosensitive Sensor Circuit Diagram

A light sensor, also known as a photosensor, is a device that transforms optical signals into electrical signals through a photosensitive element. These elements are typically sensitive to wavelengths near visible light, including both infrared and ultraviolet ranges. While primarily used for detecting light, light sensors can also serve as components in forming other types of sensors, converting various non-electrical quantities into changes in optical signals. As one of the most versatile and widely utilized sensors, the light sensor plays a crucial role in automatic control systems and non-electrical measurements.

There are numerous types of photosensors, such as photovoltaic cells, photomultiplier tubes, photoresistors, phototransistors, optocouplers, solar cells, infrared sensors, ultraviolet sensors, fiber optic sensors, color sensors, CCDs, and CMOS image sensors.

Photovoltaic Cell

Figures 1 and 2 show schematic structural diagrams and circuit diagrams of a phototransistor, respectively.

Photovoltaic cells possess several distinctive characteristics:

(1). Spectral Characteristics of Photovoltaic Cells

The spectral characteristics of a phototube refer to the relationship between the wavelength of incident light and its absolute sensitivity (quantum efficiency) under constant working voltage conditions. The spectral characteristics of photovoltaic cells largely depend on the cathode materials. Commonly used cathode materials include silver oxide photocathode, germanium photocathode, neodymium silver oxide photocathode, and multi-turn photocathode. Among these, silver oxide and germanium photocathodes are the most widely used. Figures 3 and 4 provide their respective spectral characteristics.

From the spectral characteristic curves of the phototube, we can see that phototubes made from different cathode materials exhibit varying regions of higher sensitivity. Therefore, the appropriate photovoltaic cell should be selected based on the wavelength of the measured spectrum. For instance, if the measured light contains more red light, using a silver oxide cathode photovoltaic cell would yield higher sensitivity.

(2). Volt-Ampere Characteristics of Photovoltaic Cells

The volt-ampere characteristic of a phototube refers to the relationship between the voltage (UA) between the anode and cathode of the phototube and the photocurrent (IΦ) under a certain luminous flux. Under a specific luminous flux, the phototube emits a certain amount of photoelectrons per unit time. These photoelectrons are dispersed in the space between the anode and cathode. When a voltage (UA) is applied to the anode, the photoelectrons are attracted and collected by the anode, forming a photocurrent (IΦ) in the circuit. As the anode voltage increases, more photoelectrons emitted by the cathode are collected by the anode, causing the photocurrent (IΦ) to increase. Once the anode voltage reaches a certain level, all the photoelectrons emitted by the cathode within a unit time are collected by the anode, reaching a saturated state. Beyond this point, increasing the anode voltage will no longer increase the photocurrent (IΦ).

Figure 5 illustrates a set of volt-ampere characteristics of photovoltaic cells at different luminous flux levels.

(3). Photoelectric Characteristics of Photovoltaic Cells

The photoelectric characteristics of the phototube refer to the relationship between the luminous flux (Φ) of incident light and the photocurrent (IΦ) under conditions where the anode voltage of the phototube and the spectrum of the incident light remain constant. With the anode voltage of the phototube sufficiently high to ensure the phototube operates in a saturated state, there is a linear relationship between the incident light flux and the photocurrent, as shown in Figure 6.

(4). Dark Current

If the phototube is placed in complete darkness without any light, applying a normal operational voltage still results in a weak current being generated, known as the dark current. The generation of dark current is primarily due to leakage current.

Photovoltaic cells find common application in automatic control systems, radio facsimile machines, sound films, and other photoelectric conversion devices.

Table 1 lists the technical specifications of some domestically produced photovoltaic cells.