Lab #8 Report, Physics 121 - Light Measurement and Lock-In Detection - Fall 2016 Name:
Lab Partner:
Exercise 1: Use a photodiode to sense room light
Exercise 2: Use a photodiode to measure light from a LED
Exercise 3: Characterize LED voltage, current, and light intensity relationships
(b) Use Matlab to make a plot of current vs. voltage for the LED, with axes labeled with correct units.
(c) Use Matlab to make a plot of photodiode voltage (after subtracting background signal) vs. LED current. Is the result as expected?
Exercise 4: Measure a weak photodiode signal
(c) Re-arrange the photodiode circuit so that the LED is one edge of the breadboard and pointing across to the photodiode on the other edge of the breadboard, so that only a small amount of light from the LED is falling on the photodiode. With the LED running at ~15 mA you can barely even see any the light from the LED hitting the photodiode, but can you still measure a signal? How much does the signal change when you suddenly unplug or plug in the cable at the power supply that powers the LED? What is the ratio of the LED signal to background?
Exercise 5: Characterize the noise in the photodiode measurement
(a) Write a new Labview program that acquires the photodiode signal at the maximum possible rate of 250 kHz, reads out 250 kilo samples every 1 second in a while loop. Use the “spectral measurements” function to plot the FFT (magnitude peak, result: linear, averaging on:
exponential). Average for 10 seconds and plot the FFT from 1 to 100 kHz with log scales on both axes. Include this plot in your report.
(c) Is there any difference in the FFT depending on whether you have the fluorescent desk lamp on and shining on the photodiode? Include the FFTs and discussion of any differences in your report
Exercise 6: Modulate the LED
(b) Conduct a quick biophysics experiment. What is maximum “refresh rate” for human vision?
(c) By looking at the FFT of the noise signal measured in exercise 5, choose a frequency
you detect the photodiode signal at this frequency, you can avoid most of the unwanted background noise! Include FFT plots with the LED off vs. on (and pointed up at the ceiling).
(d) Make detection of the LED signal more challenging by placing a ~1" x 1" piece of paper in front of the LED (i.e., to perhaps block more of the light from reaching the photodiode). Can you still detect the signal in the recorded FFT? Include FFT plots with vs. without the piece of paper
Exercise 7: Use lock-in detection to measure the amplitude of the weak LED signal (b) Use the “Tone Measurements” function (the one in the Express toolbar menu) to calculate the amplitude of the photodiode signal at the LED modulation frequency. Display this
re-measure the dependence of the LED intensity on current from ~5 to 15 mA, as in Exercise 3, except now you will only be detecting a tiny bit of stray light. Intensity should still be linearly proportional to LED current because this is a fundamental property of the LED that should not depend on what fraction of the light is detected. Measure signal amplitude vs. LED current for currents ranging from approximately 15 down to 5 mA in 1 mA steps by measuring the power supply voltage with the multimeter and setting it by hand to achieve the same values (=LED voltage plus resistor voltage measured previously) needed to achieve these LED currents as in Exercise 3. Plot these results with correct units labeled on the axes and include in your report.
Prelab Homework for Lab #9
Use Wikipedia/Google to look up information regarding commonly used machine shop tools. 1. What are "digital calipers" and what are they used for?
2. What are "tap and die" tools and what are they used for?
3. What drill size should be used before tapping aluminum for a 1/4 inch 20 threads per inch screw? (see: http://littlemachineshop.com/Reference/TapDrillSizes.pdf )
4. What is a "countersink" tool and what is it used for?
5. What is a "machinist square" and what is it used for?
