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Laboratory 2 Photoresistor, Photointerrupter and Transistors Pre-Lab = Required to submit your Multisim circuit files before you start the lab. Pre-lab Questions 1. (Figure Plab2.1). Explain the theory of operation of this circuit. Figure Plab2.1 Light-controlled LED 2. Use the 2N3904 datasheet provided in the UCO wiki to help you draw and label the pins on the figure below and to record the following values: IC(max) = ______ VCE(max) = _______ VBE(sat) = _________ Figure Plab2.2 2N3904 Pin-out 3. (Figure Plab2.3). In the following circuit, L1 is a light bulb rated at 0.5 A at 5 V, and the transistor’s hfe is 100. The light bulb L1 starts to glow when the current flow through it reaches 100mA, and if the current is higher than 1A, it will burn out. If the resistance of the variable resistor VR is gradually lowered from 10kΩ, at what value of VR will the light bulb start to glow? Is there a value of VR, for which the light bulb will burn out? What is it if so? Figure Plab2.3 Lamp Control Circuit Page 1 of 8 Laboratory 2 Photoresistor, Photointerrupter and Transistors Purpose • To introduce photoelectric components and transistors used as power switching devices. • To become familiar with the capability of the Basic Stamp 2SX to measure the change of resistance of a sensor • To build and experiment with a light-controlled switch Introduction Photoresistor 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 and 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. Figure 2.1 Photoresistor Transistor Figure 2.2 illustrates the nomenclature used to describe the behavior of an NPN bipolar transistor. It is a three terminal device consisting of the base, collector, and emitter. The transistor acts like a current valve by using the voltage bias across the base and emitter (VBE) to control the flow of current-emitter circuits (IC). The circuit connected to the collector and emitter along with the bias voltage dictates how much current flows. Page 2 of 8 Figure 2.2 NPN Bipolar Transistor and Nomenclature Here are some general relationships between the variables shown in Figure 2.2: Page 3 of 8 Components Qty. 1 2 1 1 1 1 1 1 1 1 1 1 Item OEM Basic Stamp 2SX microcontroller 1kΩ resistor 10kΩ resistor 330Ω resistor 220Ω resistor 470Ω resistor 0.1µF capacitor 2N3904, NPN transistor Photoresistor Photointerrupter Potentiometer LED 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 covering the cell with your hand. These two extremes will be used in calculations later on. 2. Figure Plab2.1 shows a simple ‘light-controlled-LED’. The circuit should turn-off the LED as the photo resistor is covered. 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 photo resistor has low resistance, and yet should be low enough so that the current is not enough to turn on the LED when the photo resistor has high resistance.) Build the circuit and check its function. 3. (Figure 2.3) Page 4 of 8 Figure 2.3 Transistor Switch 4. 5. Page 5 of 8 6. (Figure 2.4). Figure 2.4 shows a light-controlled-switch circuit using a 2N3904, NPN transistor. 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. As the photoresistor’s resistance decreases (as the result of an increase in light intensity), the base voltage increases. 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 increment will be amplified by the current gain of the transistor. Choose the supply voltage. The supply voltage is often a predetermined value rather than a design choice. For example, if battery is to be used, the voltage should be either 3, 6, or 9V. In the following calculation and in later circuit construction, use any voltage of your choice between 6 V to 15 V. Figure 2.4 Improved Light-Controlled Switch Using a Transistor 7. Select R1. First, determine a value of the photo resistor’s resistance (call it Ron) at which you would like the LED to be turned on. The resistance value can be that for when the photoresistor is covered or uncovered, it’s up to you. (Just make it clear in your report). The value of R1 should be such that 0.7 = V*R1/(R1+Ron). A variable resistor (trim pot) can be used so that the turn-on value can be adjusted. 8. Select Rc, the current limiting resistor. With this resistor, the collector current is limited to Imax=(V- VLED - 0.4)/RC , where VLED is the voltage drop across the LED, and 0.4 V is a typical saturation voltage between the collector and emitter. Select RC so that the LED current is limited to be less than 20 mA (preferably 5-10 mA). Using RC, construct and test the circuit. 9. (Figure 2.5). Examine the photointerrupter and look at its specifications. Build the circuit shown in Figure 2.5, using the resistors indicated. Note that a single 5V source can be used to provide both voltage signals, and the ground for the input and output circuits must be connected to be common. Measure the output voltage (Vout) with and without the beam interrupted (e.g., with a thick sheet of paper or a plastic card). What conditions (interrupted or not) correspond to the high and low states of the output? Explain why each condition results in the respective state. Why are the resistors required? Page 6 of 8 Figure 2.5 Photointerrupter 10. Build the part of the circuit in Figure 2.6 labeled ‘A’. Connect the 1k resistor to any of pins P0 through P15 on the Basic Stamp 2SX (denoted xx below). Don’t forget that you need to supply power and ground to the Basic Stamp 2SX. Figure 2.6 Light-Controlled Switch Using the Basic Stamp 2SX 11. Run the following program, and see if you can make the LED blink: 'Blinktest 'Put the date here 'Put your name here loop: high xx ‘substitute the pin number you chose in place of xx pause 500 low xx ‘substitute the pin number you chose in place of xx pause 500 goto loop Page 7 of 8 12. When you have successfully completed step 11, build the rest of the circuit shown in Figure 2.6. Connect the 220 ohm resistor to another one of the pins P0 through P15 (but different from the pin you chose in step 11, denoted yy below). 13. Run the following program: 'Light-Controlled Switch 'Put the date here 'Put your name here loop: result var word high yy ‘substitute the pin number you chose in step 12 in place of yy pause 1 RCTIME yy,1,result ‘substitute pin no. you chose in step 12 in place of yy debug ? result if result > 40 then turnON low xx ‘substitute the pin number in step 11 in place of xx goto loop turnON: high xx ‘substitute the pin number in step 11 in place of xx goto loop 14. Look at the value of the variable ‘result’ in the Debug window. What happens to ‘result’ when you cover the photoresistor? Based on the value for the variable result that is being displayed in the Debug window, you can adjust the threshold value of 40 to a value that will cause the LED to be off under ambient light conditions, but turn on when a shadow falls on the photoresistor. Experiment with the threshold value. Questions 1. For the figure 2.6, what changes need to be made to the software (note: no need to change any hardware) if you want to have the LED stay on under ambient light conditions and turn off when a shadow falls on the photoresistor? Try it. 2. (Figure 2.7). Replace the LED circuit in the figure 2.6 labeled ‘A’ with a 12V,10W lamp circuit. Use the multisim to design and simulate the 12V, 10W lamp circuit. You are only allow to use three components shown in figure 2.7 for your new circuit design. You now have the knowledge to control devices that require more power than the Basic Stamp 2SX can supply by itself. Figure 2.7 Components for 12V, 10W lamp circuit Page 8 of 8