Lab 1, Part B: Optoelectronics
This exercise is primarily intended to build your experience with LEDs and phototransistors. Concise answers to the questions in bold are to be submitted by the deadline.
0: Thinking about Optoelectronics
Before you start working on this portion of the lab, answer the following questions. Note: these will not be graded for correctness, so please do not change your answers once you complete the exercises.
0.1.1 - Why is there a 100Ω resistor in the LED circuit of 1.1.1?
0.1.2 - Describe the relation between emitter voltage (Vout) and incident light for the circuit of 1.2.1 (part 1.2)
0.1.3 - Sketch (or describe) an estimate of collector voltage, Vout, versus incident light for the slightly different phototransistor circuit shown below.
1: PLAYING WITH LIGHT
1.1 - LEDs
With a wavelength beyond that visible to the human eye, infrared light is widely used for sensing, control, and communications. In this portion of the lab, you will construct a simple infrared emitter/detector pair to send an analog signal across a short distance.
1.1.1 - Build the emitter circuit shown below. You should use an LTE-4206 IR LED, a 100Ω resistor, and a 2kΩ potentiometer. Be sure to check the polarity of the LED (the datasheet can be found here). Using a cellphone or other camera (most inexpensive digital camera sensors are sensitive to IR light, so they make a great debugging tool for IR signals), look at the front of the LED. Change the resistance of the potentiometer, and do your best to explain what is happening in the circuit to cause the change that you observe (in other words, what is changing when you turn the pot, and how does this cause a change in the light output?). Set this circuit aside while you work on part 1.2.
1.2 - Phototransistors
1.2.1 - Using an LTR-4206 IR phototransistor (the datasheet can be found here), a 1kΩ resistor, and a 2MΩ potentiometer, build the circuit shown below (step 3 will be a lot easier if you build this on a different breadboard from the emitter circuit). Be sure to check the polarity of the phototransistor before you apply power to the circuit. For additional information on phototransistor cicuits, see this application note.
1.3 - IR pairs
1.3.1 - If you orient the LED and phototransistor next-to and nearly parallel to one another, you can create a sensor capable of estimating the distance and/or the reflectivity (and thereby greyscale color) of an object. Test this out by rearranging your circuit and moving various objects in front of your newly-created sensor. You will find that there is an optimal sensing distance based upon where the centerlines of the LED and phototransistor overlap. Now, find both a white and black object, and adjust the potentiometers in your circuits to reveal a 1 Volt change in the emitter output when you place each object near the sensor's focal point. Demonstrate your reflectance sensor to one of your classmates, and record their name.
1.3.2 - Using your emitter and receiver circuits pointed at one another, you now have a simple breakbeam sensor. If you place an object between the emitter and receiver, the output voltage of the phototransistor circuit will change. Locate the emitter and receiver about 5 centimeters away from one other and try to keep them as collinear as possible. Adjust the potentiometers to create a 3 Volt change when an object is placed between the emitter and receiver. Record the potentiometer values that you found to work best. To increase the sensitivity of the phototransistor circuit, do you increase or decrease the potentiometer resistance? Why?
1.3.3 - Modify the phototransistor circuit to turn off a visible (red, green, or yellow) LED when an object breaks the beam. Sketch/describe your modified phototransistor circuit and demonstrate it to one of your classmates. Record their name.
1.3.4 (510 req, 410 e.c.) - Experimentally determine the axial cross-sectional area of constant-light output from the LED. Describe your method, and include a sketch or plot of the area.
1.4 - Cleanup & Submission
1.4.1 - Please return the potentiometers to their appropriate bins in the mini store (you may keep the LED, phototransistor, and discrete resistors).
1.4.2 - Submit either in class or to Towne 220 before 3:00 p.m. on the day of the deadline.