What Are Photosensitive Devices?
Photosensitive devices are components capable of converting optical signals into electrical signals. When paired with light-emitting devices, they enable the mutual conversion between electrical and optical signals. Common photosensitive components include photoresistors, photodiodes, and phototransistors.
I. Photoresistors
Unlike common carbon-film or metal-film resistors—whose resistance remains unchanged upon exposure to light—photoresistors exhibit resistance values that are highly sensitive to changes in light; this characteristic stems from their specific materials and structure. A photoresistor is constructed by depositing a layer of photosensitive semiconductor material onto a ceramic substrate and attaching two leads to serve as electrodes. The device features a glass window or lens that allows light to strike the thin semiconductor layer. As the incident light intensifies or weakens, the excitation level within the semiconductor changes, altering the number of charge carriers and, consequently, the device's resistance. Common types include ultraviolet, visible light, and infrared photoresistors; since they respond to different wavelengths, they must not be confused during use.
Key parameters of photoresistors include:
1) Dark resistance (RD): The resistance value of the photoresistor when not exposed to light.
2) Light resistance (RL): The resistance value of the photoresistor when exposed to light.
3) Peak wavelength: The wavelength at which the photosensitive response is optimal.
II. Photodiodes
A photodiode is a semiconductor device featuring a P-N junction. Rather than functioning as a rectifier, it converts optical signals into electrical signals, acting as a photoelectric conversion device.
Photodiodes have distinct positive and negative terminals: the lead closest to the index tab or color dot is the positive terminal (anode), while the other is the negative terminal (cathode); alternatively, the longer lead is positive, and the shorter lead is negative.
Photodiodes operate under reverse bias. In the dark, due to minimal intrinsic excitation, the reverse current—referred to as "dark current"—is very low. When illuminated, intrinsic excitation intensifies, increasing the minority carrier concentration and causing the reverse current to rise rapidly to the range of tens of microamperes; this current is known as the photocurrent. Variations in light intensity cause corresponding changes in the photodiode's photocurrent, thereby facilitating the conversion between optical and electrical signals. At a constant incident light intensity, the photodiode's reverse current remains steady and is essentially independent of the magnitude of the applied reverse voltage.
Photodiode testing method:
(1) Set the multimeter to the R×100 or R×1k range; testing is similar to that of a standard diode, and the forward resistance should be approximately 10 kΩ.
(2) Reverse the test leads to place the photodiode in a reverse-biased state. Cover the photodiode's transparent window with an object; the measured resistance should approach infinity.
(3) Remove the obstruction; the meter needle should deflect to the right, indicating a resistance of a few thousand ohms. The stronger the light, the lower the resistance value. If both forward and reverse resistance readings are infinite or zero, the component is damaged.
III. Phototransistors
While photodiodes can perform photoelectric conversion, their sensitivity is low; using a phototransistor significantly enhances this sensitivity.
Phototransistors typically feature an NPN structure where the base serves as the light-sensitive window. Consequently, most phototransistors have only two terminals—the collector and the emitter. Some models include a base terminal for temperature compensation, which can be trimmed off if not required. Regarding terminal identification: the terminal closest to the color dot mark is the emitter, the one further away is the collector, and the lead with the greater length is the base.
When using a phototransistor as a receiver, sensitivity can be improved by applying an appropriate bias current—such as supplemental illumination—to shift the device into the shallow amplification region. During installation, ensure the light-emitting diode (LED) does not obstruct the phototransistor's light-sensitive surface, as this could interfere with remote control signal reception. This method effectively boosts reception sensitivity and extends the remote control operating range.
(1) Cover the window of the phototransistor with an opaque object; in the absence of light, no current flows through the phototransistor, and the measured resistance should be infinite.
(2) Remove the opaque object and orient the phototransistor's window toward the light source; the transistor conducts, and the multimeter's needle deflects to the right—pointing to approximately 1 kΩ—with the degree of deflection indicating the device's sensitivity.
I. Photoresistors
Unlike common carbon-film or metal-film resistors—whose resistance remains unchanged upon exposure to light—photoresistors exhibit resistance values that are highly sensitive to changes in light; this characteristic stems from their specific materials and structure. A photoresistor is constructed by depositing a layer of photosensitive semiconductor material onto a ceramic substrate and attaching two leads to serve as electrodes. The device features a glass window or lens that allows light to strike the thin semiconductor layer. As the incident light intensifies or weakens, the excitation level within the semiconductor changes, altering the number of charge carriers and, consequently, the device's resistance. Common types include ultraviolet, visible light, and infrared photoresistors; since they respond to different wavelengths, they must not be confused during use.
Key parameters of photoresistors include:
1) Dark resistance (RD): The resistance value of the photoresistor when not exposed to light.
2) Light resistance (RL): The resistance value of the photoresistor when exposed to light.
3) Peak wavelength: The wavelength at which the photosensitive response is optimal.
II. Photodiodes
A photodiode is a semiconductor device featuring a P-N junction. Rather than functioning as a rectifier, it converts optical signals into electrical signals, acting as a photoelectric conversion device.
Photodiodes have distinct positive and negative terminals: the lead closest to the index tab or color dot is the positive terminal (anode), while the other is the negative terminal (cathode); alternatively, the longer lead is positive, and the shorter lead is negative.
Photodiodes operate under reverse bias. In the dark, due to minimal intrinsic excitation, the reverse current—referred to as "dark current"—is very low. When illuminated, intrinsic excitation intensifies, increasing the minority carrier concentration and causing the reverse current to rise rapidly to the range of tens of microamperes; this current is known as the photocurrent. Variations in light intensity cause corresponding changes in the photodiode's photocurrent, thereby facilitating the conversion between optical and electrical signals. At a constant incident light intensity, the photodiode's reverse current remains steady and is essentially independent of the magnitude of the applied reverse voltage.
Photodiode testing method:
(1) Set the multimeter to the R×100 or R×1k range; testing is similar to that of a standard diode, and the forward resistance should be approximately 10 kΩ.
(2) Reverse the test leads to place the photodiode in a reverse-biased state. Cover the photodiode's transparent window with an object; the measured resistance should approach infinity.
(3) Remove the obstruction; the meter needle should deflect to the right, indicating a resistance of a few thousand ohms. The stronger the light, the lower the resistance value. If both forward and reverse resistance readings are infinite or zero, the component is damaged.
III. Phototransistors
While photodiodes can perform photoelectric conversion, their sensitivity is low; using a phototransistor significantly enhances this sensitivity.
Phototransistors typically feature an NPN structure where the base serves as the light-sensitive window. Consequently, most phototransistors have only two terminals—the collector and the emitter. Some models include a base terminal for temperature compensation, which can be trimmed off if not required. Regarding terminal identification: the terminal closest to the color dot mark is the emitter, the one further away is the collector, and the lead with the greater length is the base.
When using a phototransistor as a receiver, sensitivity can be improved by applying an appropriate bias current—such as supplemental illumination—to shift the device into the shallow amplification region. During installation, ensure the light-emitting diode (LED) does not obstruct the phototransistor's light-sensitive surface, as this could interfere with remote control signal reception. This method effectively boosts reception sensitivity and extends the remote control operating range.
(1) Cover the window of the phototransistor with an opaque object; in the absence of light, no current flows through the phototransistor, and the measured resistance should be infinite.
(2) Remove the opaque object and orient the phototransistor's window toward the light source; the transistor conducts, and the multimeter's needle deflects to the right—pointing to approximately 1 kΩ—with the degree of deflection indicating the device's sensitivity.
Sep 22,2026