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4photoresistor-atmel

4photoresistor-atmel - Photoresistor Laboratory PH-1...

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Photoresistor Laboratory PH-1 © San José State University Dept. of Mechanical and Aerospace Engineering rev. 3.1 18SEP2007 Photoresistor, Transistor, and LED’s Purpose: To introduce photoresistors, LED’s, FET’s, and transistors used as power switching devices To become familiar with the capability of the Atmega 128 to measure the change of resistance of a sensor using analog-to-digital conversion. To build and experiment with a light-controlled switch Components: Qty. Item Qty. Item 1 10 k Ω resistor 1 22 k Ω resistor 1 Solderless Breadboard 1 470 Ω resistor 1 Photoresistor 1 220 Ω resistor 1 2N3904, NPN transistor 1 red or green LED 1 Atmel Atmega 128, STK500 & STK 501 interface boards, and programming cable. Introduction: A photoresistor is simply a resistor whose resistance depends on the amount of light incident upon it. They are used to make light-sensitive switching devices. Photoresistors are often made from cadmium sulfide (CdS). The resistance of a CdS photoresistor varies inversely to the amount of light incident upon it. In other words, its resistance will be high in the dark and low in the light. The LED behaves like an ordinary diode except that when it is forward biased, it emits light. The LED’s forward voltage drop is higher than an ordinary diode. Typical LED’s require 5 to 15mA to reach full brightness, but are not designed to handle more than about 20 mA of current (though some can handle upwards of 80 mA or more). You will therefore always need to provide a resistor in series with an LED to limit the current to about 20 mA or less, or else you will burn it out. Also, don’t make the mistake of trying to substitute an LED where a standard diode is called for! Look at the schematic diagram to see which kind of component is needed. Procedure 1. Measure the photo-resistor’s resistance in the ambient lighting of the lab. Once this is recorded, repeat the measurement, only this time, cover the cell with your hand. These two extremes will be used in calculations later on. 2. To verify the behavior of the LED, construct the circuit shown in Figure 1, and vary the supply voltage between 1 to 5 volts at 1-volt increments. At each voltage, measure the voltage across the LED and the 470 Ω resistor and enter the values into the following table. The LED current can be calculated by applying Ohm’s law across the resistor. A similar table should be entered into the lab report with all voltage values and comments. Remember I=R/V for current + V 470 Ω LED + - side top anode cathode flat or notch + -
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