temporal response of a photoresistor
DESCRIPTION
Temporal Response of a PhotoresistorGladys Regalado1, May Ann Tenorio2 National Institute of Physics, University of the Philippine, Diliman Quezon City 1 [email protected] 2 [email protected] temporal response of the photoresistor was observed. LED was used to act as an excitation source. The system is a first order system. For the square wave input, the maximum flicker frequency that can be detected reliably is up to 100 Hz, and for the sine wave, it is up to 1KHz. ThereTRANSCRIPT
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Temporal Response of a Photoresistor Gladys Regalado1, May Ann Tenorio2
National Institute of Physics, University of the Philippine, Diliman Quezon City [email protected]
[email protected] Abstract The temporal response of the photoresistor was observed. LED was used to act as an excitation source. The system is a first order system. For the square wave input, the maximum flicker frequency that can be detected reliably is up to 100 Hz, and for the sine wave, it is up to 1KHz. There was an observed change in phase for the sine wave input as the frequency is varied, but none with the square wave. Given constant excitation intensity, the transfer function is also constant, since the resistance of the photoresistor only changes with varying light intensity.
1. Introduction A photoresistor is a device that increases its resistance with decreasing light intensity. They are commonly used for automatic activation of lamps, light detectors, etc [1]. In this experiment, the temporal response of a photoresistor to an excitation by an LED will be observed and characterized. 2. Experimental Set up and Methodology Figure 1 shows the testing circuit used in the experiment. A newspaper was used to cover the whole set up to limit the ambient light that comes from the surroundings. 3. Results and Discussion
Figures 2-5 shows the temporal response of a photoresistor to an excitation provided by an LED. The resistor used for the LED circuit has a value of 1kΩ, with an input voltage of 10 V. This resistor value at the said input voltage ensures to limit the current passing through the LED to a safe value. The resistor used for the photoresistor has a value of 15kΩ, with an input voltage of 15 V. This resistor value limits the current that passes through the photoresistor, thereby preventing it from burning due to excessive current.
The response of the photoresistor was observed for a square wave and a sine wave input. For the square wave input, the maximum flicker frequency that can be detected reliably is up to 100 Hz, and for the sine wave, it is up to 1KHz. There is an observed change in phase as the frequency is varied for the sine wave. However, such change is not observed for the square wave.
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Figure 2. Input and output signal for a square wave (left) and a sine wave (right) at 1 Hz.
Figure 3. Input and output signal for a square wave (left) and a sine wave (right) at 10 Hz.
Figure 4. Input and output signal for a square wave (left) and a sine wave (right) at 100 Hz.
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Figure 5. Input and output signal for a sine wave at 1KHz.
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Figure 6. Gain plots for a square wave input (a) and sine wave input (b) function.
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Figure 7. Phase plot for the sine wave input function.
The gain plot and phase plot was obtained for the two input functions. Since there is no observed change in phase for the square wave, it was not shown. Figure 6 shows the gain plot of the two input functions and figure 7 shows the phase plot for the sine wave. From the gain plot, it can be determined that the system is a first-order system. It lacks the characteristic peak that can be found in second-order systems. Given a constant excitation intensity, the transfer function of the photoresistor is also constant, since its resistance will only vary if the light intensity is varied. Conclusion The temporal response of the photoresistor was observed, with an LED acting as an excitation source. From the gain plot, it can be said that the system is of the first order. For the square wave input, the maximum flicker frequency that can be detected reliably is up to 100 Hz, and for the sine wave, it is up to 1KHz. Given a constant excitation intensity, the transfer function is also constant. References [1] Retrieved from http://www.electronicsarea.com/photoresistor.asp (Date Accessed: January 31, 2010) [2] Activity 5 – Photoresistor Laboratory Manual. Applied Physics 185
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