LAB PROJECT 2. Lab Exercise

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1 LAB PROJECT 2 Objective Investigate photoresistors, infrared light emitting diodes (IRLED), phototransistors, and fiber optic cable. Type a semi-formal lab report as described in the lab manual. Use tables where appropriate and use Excel or similar program for plots. Parts Needed Cable kit, photoresistor, LED, 555 timer, IRLED, IR phototransistor, fiber optic cable, 6.3 volt incandescent light, breadboard. Photoresistor Lab Exercise 1. Determine the resistance of a photoresistor a. Using a multimeter, measure the resistance of the photoresistor under normal room light conditions, also called ambient light. b. Cover photoresistor and measure dark resistance. c. Using the supplied incandescent light bulb and powered by 6 Vdc, place it as close as possible to the photoresistor and measure resistance of the photoresistor. d. Move the photoresistor in 1 cm increments away from the light source and record data. In your report, plot resistance vs distance. Be sure to label plot and axes. Is the plot linear, logarithmic, power, etc.? What is the maximum distance that a response is evident? e. With the incandescent light source next to the photoresistor, vary the light voltage from to 6 volts in ½ volt increments and plot light source photoresistor resistance vs current. Is the plot linear, logarithmic, power, etc.? 1

2 2. Configure a 555-timer for approximately 1- Hertz astable operation, selecting resistors Ra and Rb according to the chart in the specification sheet. Use a 4% duty cycle for your calculations. a. Use an LED plus resistor as the output load. Calculate the output LED resistor value so that 2mA flow through the LED. Connect output load to the 555 timer as shown in Figure 1. +Vcc RL R visible LED 555 Timer 3 Figure timer output load connected to pin 3 3. Replace resistor Rb with the photoresistor and observe the results of the output of the timer with the photoresistor dark (covered) and under ambient lighting. Observe the results with the incandescent light adjacent to the photoresistor. What can you say about the frequency and duty cycle? Infrared Diode and Infrared Transistor 1. The diode test function on the multimeter is a 1 ma current source. It will indicate the voltage drop across a diode or LED (Light Emitting Diode). Sometimes, however, the LED will be out of range of the diode test and no observable changes will occur. Although we cannot see infrared light, digital cameras and camera phones will respond. If the IRLED is attached to the diode test in the correct polarity, it will light and it can be viewed with a digital camera. 2. Another way to reverse engineer the IRLED and IR Phototransistor. Using the diode test function of the multimeter, test the IRLED and the IR phototransistor to determine the anode and cathode and distinguish the IRLED from the IR phototransistor. 2

3 a. Distinguish the IRLED from the IR Phototransistor by comparing the diode measurement value using the 6.3 volt incandescent light. The IR phototransistor responds to light and will show a reading in one direction with the diode test. For this transistor, the anode that is now identified is called the collector and the cathode is called the emitter. b. For the IRLED, the leads will register open in one direction and usually register a value on the meter in the other direction. The IRLED does not respond to the incandescent light in either direction. If there is no response to incandescent light, then it is probably the IRLED in which case the flat spot on the casing is the cathode, as in the visible LED s. 3. Connect 5-volts through a 1k resistor to the collector of the IR phototransistor and connect the emitter of the phototransistor to ground, as shown in Figure 2. Figure 2. Phototransistor circuit. 5Vdc V1 R1 1k, 1k, 56k Vout collector phototransistor emitter 4. With R1 = 1k, measure Vout (the collector voltage) with the IR phototransistor dark (covered) and under normal room lighting. Next observe and record the Vout with the incandescent light adjacent to the IR phototransistor. Perform the same test with R1 = 1k and R1 = 56k. 5. What can you conclude about Vout and the resistance? Explain the results. 3

4 6. Using a 12-volt power supply, determine the value of resistance R2 needed for 4 ma through the IRLED. Connect the IRLED as shown in Figure 3. Figure 3. IRLED circuit. R2 V2 IRLED a. Set up the IR phototransistor. Using R1 as 1k, 1k, and 56k as shown in Figure 2, measure Vout (the collector voltage) with the IRLED pointed directly in line and close to the IR phototransistor. Assuming the IRLED light source is constant (keeping the same distance and angle from the IR phototransistor), what happens when the IR phototransistor resistance R1 is changed? 7. Using the 1k resistor for R1 in Figure 2, measure the maximum distance between the IR diode and the IR transistor where a response is evident. Now change R1 to 56k and record the maximum distance. What can you conclude about why different resistance values would be used? 4

5 Fiber Optics 1. Using the 2-meter fiber optic cable, place one end at the end of the incandescent bulb and observe the opposite end of the cable. You should see light through the cable. Infrared light can also be transmitted through the cable, although our eye cannot detect it. (Cameras will). 2. Set up the circuit in Figure 4. If everything is correct, the infrared beam through the fiber optic cable will light the visible LED. Be sure to align the fiber optic cable so that the fiber optic cable is directly on top of the IRLED and IR phototransistor. Figure 4. IRLED and IR transistor circuit with LED, connected by fiber optic cable. V2 IRLED R2 (4 ma) fiber optic cable R1 1k visible LED V1 phototransistor 5