Circuits, Oscilloscopes, & Measuring the Speed of Light

Circuits, Oscilloscopes, & Measuring the Speed of Light In this lab you will use laser pointers, integrated circuits, and an oscilloscope to measure t...
Author: Silas Harrison
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Circuits, Oscilloscopes, & Measuring the Speed of Light In this lab you will use laser pointers, integrated circuits, and an oscilloscope to measure the speed of light. At your lab station, there are 2 pre-constructed circuits (Figure 1), a highbandwidth function generator (Figure 2), and an oscilloscope (Figure 3).

Figures 1, 2, & 3: Two circuits, a function generator, and an oscilloscope The first circuit we will use in today’s lab is called a “one-shot” pulse generator. It functions by generating a distinct pulse whenever its input trigger button is pressed. There are two chips in this circuit, represented in their circuit diagrams (Figures 4 & 5) as U1 and U2. U1 generates pulses of 6V, with a duration of length R1*C1.

Figures 4 & 5: The “one-shot” circuit (larger versions at end of document)

• In milliseconds, what will be the duration of output pulses from U1? • U2 is an OpAmp. What is its function as the circuit is designed? Connect the probe for Channel 1 on the oscilloscope to two points on the circuit. The first point should be Ground, which is accessed anywhere on along the blue line of pins in this circuit. The other should be the output of the “one-shot” chip (U1), before going to the OpAmp U2. • What row of pins in this circuit corresponds to U1? Turn on the oscilloscope, access the Trigger Menu, and set the Trigger to “Auto” mode and “Rising Edge” trigger. Now set the scope to trigger on CH1. Adjust the V/div knob for Channel 1 to 2.00 V/div – this sets the scale of the y-axis. Adjust the sec/div knob to an appropriate value. • According to your previous calculation of pulse duration, what is an appropriate value for sec/div? Now, the probe for Channel 1 should read Ground. Adjust the Trigger Level to just above the current probe reading. Press the “Clear” button on the circuit. Next, press the “Trigger” button. A pulse should appear on the scope. What is its duration? We now wish to compare the output of the “one-shot” circuit (which is currently visible on Channel 1 of the oscilloscope) to the output of the OpAmp. As with Channel 1, hook up the probe for Channel 2 of the oscilloscope to the output of the OpAmp and to Ground. Leaving the settings related to the trigger alone, adjust the V/div knob for Channel 2. • What row of pins corresponds to the output of the OpAmp? • According to your understanding of this circuit, what is an appropriate setting for V/div for Channel 2, the output of the OpAmp, U2 Now, trigger the one-shot again. You should be able to compare the output of U1 (the oneshot chip) and U2 (the OpAmp) directly. The pulse from the one-shot, shown on Channel 1, should look clean and well-defined. How precise is its leading edge? Use the oscilloscope to zoom in on the x-axis to observe the risetime of the pulse – ie., how long does it take the pulse to go from low to high?

• How long is the rise time for the pulse coming out of the OpAmp? How far can light travel in this amount of time? The other circuit we will use in today’s lab is a Silicon PhotoDiode with OpAmp. (See Figures 6 and 7.) A Silicon Photodiode is a simple device that responds to light by creating a voltage across its leads. Using the oscilloscope, we can measure the intensity of light coming into the photodiode just by measuring the voltage across its leads.

Figures 6 & 7: The Silicon PhotoDiode circuit (larger versions at end of document) Hook up the probe for Channel 3 on the oscilloscope to the output of the OpAmp and to Ground. Use a laser pointer to test that the photodiode (and thus the OpAmp) changes its voltage when a beam is shone directly into it. • What is the gain for the OpAmp in this circuit? Once you can see that the Silicon PhotoDiode circuit responds to light from the laser pointer, you’re ready to put both circuits to use in measuring the speed of light.

Measuring the Speed of Light – Part 1 Attach a laser pointer to the output of the OpAmp from your first circuit, as shown in Figure 8. Note that electrical tape has been used to hold down the laser pointer’s button, and the case is connected to Ground. After connecting the laser to your circuit, you should be able to fire the laser pointer using the trigger button on your circuit. (More photographs of the fully connected circuit are shown at the end of the document.)

Figure 8: Laser-pointer attached to circuit Aiming the laser pointer directly at the Silicon PhotoDiode of your other circuit, use the oscilloscope to measure the time between firing pressing the trigger button on your circuit, and seeing a response in the Silicon PhotoDiode. All the oscilloscope settings should be the same as before: You’ll be triggering on Channel 1 (output from the one-shot), and comparing to the output of Channel 3 (output from the OpAmp with the photodiode). The result of this measurement should look like the first oscilloscope photograph found at the end of the document. Using the mirrors and alignment equipment we’ve provided (see Figure 9), bounce the laser’s beam off of a mirror on the other side of the room. Now use the same photodiode to measure the time between triggering the one-shot and receiving the pulse. Note, the switch on the one-shot’s board turn the laser on constantly; this can be very helpful in aligning the mirror.

Figure 9: Laser and mirror alignment tools • How much more time is there between the trigger and the received pulse after sending the beam across the room? • How fast is the speed of light, according to the data you have collected?

Measuring the Speed of Light – Part 2

Now, we will measure the speed of light again, using a beam-splitter to send our laser beam down two different paths simultaneously. The setup is a recreation of an important experiment performed in the early 20th century, by the scientists Albert Michelson and Edward Morley, with slight modification. Detach the laser pointer from the one-shot circuit and connect it to the function generator (see Figure 10 & 11). Most of the settings on the function generator should already be correct. It’s worth noting that it is possible to permanently damage a laser pointer by hooking it up to a function generator with the wrong settings. Note that the function generator’s signal is also connected to Channel 4 of your oscilloscope before reaching your laser. Use the Trigger Menu to set CH4 as the Trigger.

Figures 10 & 11: Connecting the function generator to the laser pointer Now, turn on the function generator (the switch is on the back). The laser pointer should turn on. On the oscilloscope, adjust the trigger level knob, the CH4 V/div knob, and the sec/div knob until the signal going into the laser pointer is clearly displayed on the screen. The shape should be sinusoidal. Adjust the “Frequency Multiplier” knob on the function generator. By turning it down a few clicks, you should be able to see that the the sine wave of the function generator is modulating the intensity of the light coming out of the laser pointer. Turn the “Frequency Multiplier” knob all the way back to the maximum – this increases the frequency of the modulation to a speed faster than the human eye can detect.

Now, recreate the setup from Figures 12, 13, and 14. Some advice in recreating the setup. 1. Align the long beam-path to the Silicon PhotoDiode first. 2. Use the oscilloscope to measure the response in the Silicon PhotoDiode as you are aligning the beam 3. When aligning the second, shorter beam-path, block the first beam path, so that you can again use the oscilloscope to measure your alignment.

Figures 12, 13, & 14: Splitting the beam down two paths. (Note: the far mirror outside the area of these photographs. It should be at least 15 feet away from the beam-splitter) Once you have aligned the setup, check the intensity of the Silicon PhotoDiode’s response to the two beams seperately. (Use you hand to block one path, while measuring the other on the oscilloscope.) In the likely event that the longer beam-path has a lower intensity, adjust the alignment of the mirror for the shorter beam-path to reduce its intensity. Once the two beams both seem to be reaching the Silicon PhotoDiode, providing nearly equal intensity in response, you can proceed to make a measurement. Now that you have one sinusoidal beam, split down two paths, think about the light that the Silicon PhotoDiode will see.

• What would the Silicon PhotoDiode see if the length of the two beams were the same? • Measured in units of period, “T”, how much would you have to delay the one beam to get the two beams exactly out of phase? • What would the response of the Silicon PhotoDiode look like if the two beams arrived exactly out of phase? • What is the difference in length of the two beam-paths you have created? • As frequency rises, period falls ( f = 1 / T ). What frequency would you estimate is high enough (and the period low enough), that the two beams show up at your Silicon PhotoDiode out of phase? • Adjusting the frequency knob on your function generator, do you see the behavior you expect? Using the oscilloscope, can you identify the frequency at which the two beams are showing up out of phase? Using only this number (and the difference in beam-paths in your setup), what is your measurement of the speed of light?

The “one-shot” circuit diagram

The “one-shot” circuit: laser & batteries are not attached

The Silicon PhotoDiode circuit

The Silicon PhotoDiode circuit: Batteries are not attached

The “one-shot” laser pulser circuit fully setup. (The laser is not shown attached)

The Silicon PhotoDiode circuit fully setup. (Scope is not attached)

Two scope traces from Activity 1: Measuring the speed of light at different path lengths. Yellow: Output of U1 (the MM74HC123AN) on the One-Shot Circuit (pin-row 4 G-J) Blue: Output of U2 (the AD8011) on the One-Shot Circuit (pin-row 16 A-D) Purple: Output of U1 (the AD8011) on the Silicon PhotoDiode Circuit (pin-row 10 G-J)