When electromagnetic radiation, such as, infrared light, visible light, or ultraviolet (UV) light, strikes a photoconductive material, the resistance of the material decreases. This occurs because the electrons in the valence band of the photoconductive material are excited by the light and move to the conduction band, which increases the material’s conductivity. The amount of the resistance change depends on the light density. Most photoresistive sensors are made of semiconductor materials, such as cadmium sulfide (CdS).
Characteristics of Photoresistor Sensor
A photoresistor is also called a light-dependent resistor (LDR), photoconductor, or photocell since its resistance changes as incident light intensity changes. When placed in the dark, its resistance is as high as 1 MΩ and then falls to 400 Ω when exposed to bright light.
Key performance characteristics of photoresistive sensors are as follows:
Responsivity Rd: the ratio of detector output to light input. It measures the effectiveness of the detector in transducing electromagnetic radiation to electrical voltage or current. If the sensor’s output is voltage, Rd is the ratio of the root mean square (RMS) of the output voltage VRMS to the incident radiant power Φe (in watts):
Rd = VRMS/Φe (2.37)
If the sensor’s output is current, Rd is the ratio of the RMS of the output current IRMS to the incident radiant power Φe (in watts):
Rd = IRMS/Φe (2.38)
Spectral response curve: a plot of the sensitivity as a function of wavelength as shown in Figure 1.
Noise equivalent power (NEP): the minimum detectable signal level defined as the radiant power that produces an output voltage equal to the noise voltage of the sensor:
NEP = (Ee Ad)/(Vs/Vn)
Where Ee is the power density at the surface of the sensor in Wcm−2, Ad is the sensitive area of the photodetector in cm2, and VS /Vn is the signal to noise ratio. NEP has a unit watt (W).
Detectivity D: a measure of the intrinsic merit of a sensor material. It is a function of the sensitive area of the photodetector Ad (cm2 ), bandwidth of the measuring system B (Hz), and NEP (W):
D = (AdB)1/2/NEP
The unit of the detectivity D* is cm ⋅ Hz1/2 ⋅ W−1.
D is often used to compare different types of detectors. The higher the value of D , the better the detector.
Manufacturers often list D followed by three numbers in parentheses, for example, D (850, 900, 5), meaning that the measurement was made at a wavelength of 850 nm, with a chopping frequency of 900 Hz and a bandwidth of 5 Hz.
Quantum efficiency (QE): the effectiveness of a photodetector in producing electrical current when exposed to radiant energy. QE (in percentage) can be described by
QE = Number of electrons ejected × 100/Number of incident photons
Photoresistive Sensor Design
Figure 2 illustrates the typical construction of a photoresistor. To increase “dark” resistance values and reduce “dark” current, the resistive path is often designed as a zigzag pattern across the ceramic substrate.
Materials used in photoresistors include cadmium sulfide (CdS), lead sulfide (PbS), cadmium selenide (CdSe), lead selenide (PbSe), and indium antimonide (InSb).
CdS is the most sensitive photoresistor to visible light. Its resistance value can change from many megaohms in the dark to several kiloohms when exposed to light. PbSe is the most efficient in near-infrared light photoresistor. CdS, PbSe, and CdSe can be made to operate at light levels of 10−3– 103 footcandles. CdS photoresistor sensors are of very low cost. They are often used in autodimming, darkness, or twilight detection for turning street lights ON and OFF, and for photographic exposure meters.
Photoresistors, compared to photodiodes or phototransistors, respond relatively slow to light changes. For example, a photoresistor cannot detect the characteristic blinking of fluorescent lamps (turning ON and OFF at the 60 Hz power line frequency), but a phototransistor (which has a frequency response up to 10,000 Hz) can.
If both sensors are used to measure the same fluorescent light, the photoresistor would show the light to be always ON and the phototransistor would show the light to be blinking ON and OFF. Thus, phototransistors can be used to detect an incandescent lamp that acts as a timing start indicator.
Photocells are commonly used to find certain objects through measuring the reflectivity of a light source such as a red LED (light-emitting diode), but they are sensitive to ambient lighting and usually need to be shielded.
Photoresistive Sensor Applications
Photoresistors are generally low cost, small size, fast response, high sensitivity, and ease of use. They are broadly applied in light, radiation, and fire detectors; motion sensing; light intensity measurement; and inventory bar code reading.
Figure 3 shows a photoresistive flame detector. It contains an anode and a UV-sensitive photoresistive component (usually as a cathode). When UV light from a flame is present at the photocathode, photoelectrons are excited and emitted from the cathode and move toward the anode under the voltage provided by the batteries. A readout circuit measures charges moving toward the anode to indicate the presence of the flame.
Figure 4 is a light-controlled LED circuit. It will turn the LED ON when the CdS photoresistor is exposed to light, or turn the LED OFF when the photoresistor is blocked from the light. The control mechanism is the variation of the transistor’s base voltage VBE or base current IB. Once the transistor is ON or ACTIVE, the collector current IC controls the LED. The LED could be replaced with a relay or motor to actuate other circuits or devices.
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