Photoresistor, Transistor, and LED’sFigure 2. Light-controlled LEDPhotoresistor Laboratory PH-1 ©San José State University Dept. of Mechanical and Aerospace Engineering rev. 3.2 17SEP2010 Photoresistor, Transistor, and LED’s Learning Objectives: By the end of this laboratory experiment, the experimenter should be able to: • Explain how a photoresistor works • Describe the voltage-current relationship for an LED • Build a circuit that includes an LED, photoresistor, and transistor and interface the circuit to a microcontroller to create a light controlled switch • Write a program for the ATmega16 to control and modify the functionality of a light controlled switch Components: Qty. Item Qty. Item 2 10 kΩ resistor 1 470 Ω resistor 1 Solderless Breadboard 1 220 to 270 Ω resistor 1 Photoresistor 1 red or green LED 1 2N3904, NPN transistor 1 STK500 with Atmel ATmega16 Introduction: A photoresistor is simply a resistor whose resistance depends on the amount of light incident upon it. Photoresistors are used to make light-sensitive devices, and 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 higher at low light levels (in the dark) and lower at high light levels (in the light). A light emitting diode (LED) behaves like an ordinary diode except that when it is forward biased, it emits light. An LED’s forward voltage drop is higher than that of an ordinary diode. Typical LEDs (the two-wire leaded “jelly bean” type) require 5 to 15mA to reach full brightness, but are not designed to handle more than about 20 mA of current (though some designed specifically for lighting applications can handle upwards of 1A 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 photoresistor’s resistance in the ambient lighting of the lab. How stable is it? 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. Measure the actual resistance of the 470 Ω resistor and record your reading. Vary the supply voltage from 1 to 5 volts using 1-volt increments. At each voltage, measure the voltage across the LED and the 470 Ω resistor using the DMM, and enter the values into the following table.Photoresistor Laboratory PH-2 ©San José State University Dept. of Mechanical and Aerospace Engineering rev. 3.2 17SEP2010 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 Ohm’s law for calculating the current through a resistor: I = V/R, where V is the voltage across the resistor. +VS470 Ω LED + - Side view Top view anode (A) cathode (C or K) flat or notch +-VR VLED Figure 1. LED and typical circuit. Note that the anode lead is longer than the cathode. Sometimes there is a flat on the cathode side of the LED to help you distinguish anode from cathode. With voltage sources above the maximum forward voltage of the LED, you must always use a resistor in series to limit the current through the LED. Table 1. LED circuit measurements (Refer to Figure 1) VS, V VLED, V VR, V Current, mA Comment on LED brightness 1 2 3 4 5 Figure 2 shows a simple ‘light-controlled-LED’. The circuit should turn-off the LED as the photoresistor is covered. Explain the theory of operation of this circuit. Based on the information obtained above, what is a good supply voltage to use? (Hint: V should be high enough so that enough current flows through the LED when the photoresistor has low resistance, and yet should be low enough so that the current is not enough to turn on the LED when the photoresistor has high resistance.) Build the circuit in Figure 2, and check its function. Describe its operation. CdSV CdS VRC Figure 2. Light-controlled LED Figure 3. Light-controlled using a transistor “switch”Photoresistor Laboratory PH-3 ©San José State University Dept. of Mechanical and Aerospace Engineering rev. 3.2 17SEP2010 The Light-Controlled Switch Using a Transistor A transistor can be added to the light-controlled-switch circuit to improve its sensitivity and to eliminate the ‘half-on-half-off’ state of the LED. A rudimentary circuit to do so is shown above in Figure 3 (you don’t have to build this circuit). Here the photoresistor controls the amount of current flowing into the base of the transistor, which in turn controls the collector current of the transistor, thus controlling the current through the LED. Unfortunately, this circuit may not function properly, because when the photoresistor is in the dark state, and the LED is supposed to be turned off, the base current may be large enough that the LED may stay lit! Prove this (not now, but when you write your report), by calculating the collector current for the circuit in Figure 3 when V=10 V, RCdS=100 kΩ, Rc=220 Ω and hfe=100.Figure 4 shows an improved circuit. This is the circuit that you will build and experiment with next. CdS R1VS RcB C E CBE Figure 4. Improved light-controlled switch using a 2N3904 transistor. With the flat side of the transistor facing away from you, the pins from left to right are: collector, base, and emitter. (Transistor pinout source: http://www.fairchildsemi.com/ds/2N/2N3904.pdf) With a properly selected resistor R1, the voltage at the base of the transistor in the dark state is less than 0.7 V, and therefore the transistor is in the cut-off state. Since the transistor is cutoff, no current flows from its collector to its emitter, so the LED will be off. As the photoresistor’s resistance decreases (as the result of an increase in light intensity), the voltage at the base increases due the voltage divider formed by R1 and the photoresistor. Once the base voltage reaches 0.7 V, the base current starts to flow, and any further decrease in the photoresistor’s resistance causes an increase of base current. This base current