Scopes, Breadboards, Sensors

1) The Oscilloscope

An oscilloscope can do many things and have many specific features, but it is essentially a device that records and then visually displays information about voltages for humans to understand. In most instances, the voltage is plotted as a function of time¹. This functionality is extremely useful for analyzing and designing complicated electrical circuits. The usefulness of an oscilloscope comes down to two major problems that it solves for us:

• Humans can't inherently quantitatively assess most electrical phenomena. An oscilloscope captures electrical information and converts it to a visual format that is easy for a human to consume.
• Humans are comparatively slow creatures compared to most electronics. On our good days we can detect and process events that happen on the order of tens to hundreds of milliseconds in duration, whereas many modern electronics have events that take place over milli-, micro-, nano-, or even picoseconds. An oscilloscope can capture signals at a very high rate and then display snapshots of them that lets us, the humans, process and analyze them.

With these ideas in mind, let's look at our specific oscilloscope.

1.1) DSOX1204G

The 6.200 oscilloscopes come from Keysight (Thank you Keysight!). Do not abuse them. Take care of them. Treat them like you would treat your firstborn child or a cherished pet. Keysight makes good stuff so they're resilient, but still, respect these machines.

At first glance, the oscilloscope can be overwhelming. There's a screen and a whole lot of buttons with archaic names and words. Don't worry; most of them will start to make sense after using them for a while (and we'll get a lot of practice with them this semester!).

The first thing you'll notice is the screen where all information is displayed. Basic buttons you should know about:

• On/Off Button: You can turn on/off the oscilloscope with this. Powering the oscilloscope can take up to a minute, so be patient. If you want to turn the scope off, just push the power button again.
• Auto Scale: This is a bad button that we don't recommend pushing ever. That might be a slight overstatement, but it's mostly true; Auto Scale makes the scope try to set a zoom level based on measurements of the signal, but it can often give misleading results by zooming in on the wrong part of the signal. Use with caution!
• Default Setup: This will return the scope to a default/known mode (good to do after power up).
• Horizontal Scale: Allows you to zoom in or out in the time axis and adjust offets.
• Vertical Scale: Allow you to zoom in or out in the vertical (voltage) axis and adjust offsets.
• Run/Stop: Essentially a Play/Pause button.
• Screen Selection Controls: A series of buttons and a turny/clicky knob that allows you to select the appropriate settings for a given menu.
• Measurements Menu: Brings up the Measurements Menu.
• Cursors Menu: Brings up the Cursors Menu.
• Trigger Control and Settings: Brings up the Trigger Menu
• Channel Select: Brings up settings for each channel (also used to turn on/off individual channels)
• Function Generator Output: Our scope can generate reference and test signals and output them via this connector.
• Channels 1,2,3,4 Inputs: Signals get into the scope for measuring via the probes that are connected to these channels.
• Wave Gen Button (Menu): Brings up the Wave Generator menu

Go ahead and turn on the oscilloscope using the On/Off button. It will take a little while, so be patient. After it has booted, press the Default Setup button to place the scope back into a standard mode.

1.2) Probes

• The oscilloscope has four input channels on it. They are color-coded so you know which is which since the cables often get tangled.
• Special oscilloscope probes are used to connect to electrical signals and convey them into the oscilloscope channels.
• Each probe has two conductors, a ground conductor and the signal conductor, with both routed back to the oscilloscope.
  • The central conductor is accessed by pushing back gently on the probe tip and connecting the grabby part to a wire or conductor of interest
  • The ground connector is the small side alligator clip and must always be tied to the ground of the circuit in interest! You cannot put it at any arbitrary point in your circuit like you can with some other equipment. Ground must be connected to circuit ground!
• Never remove the ground clip from the oscilloscope probe, since it will inevitably get lost. There is a special place reserved in hell for people who remove ground clips from oscilloscope probes.
• There is a slider setting that can add a 10X scaler or 1X scaler on the signal (for very large signals). Always leave this slider on 10X.

2) Our First Signal

To get started, we're going to generate a signal using the Wave Generator menu which is sent out via a "BNC-to-alligator clip" cable. We will then read that signal back in to Channel 1 on our oscilloscope by bridging it together on the breadboard (you can get a breadboard from the cart in the far corner of the room, ask a staff member if you're having trouble finding it).

An electrical schematic of what we want to do is shown below:

2.1) Breadboards

In order to create this circuit bridge, we will use a breadboard. A breadboard is an electrical prototyping device that contains rows of springs intended for holding component leads. Particular groups of these springs are connected together underneath by conductors. As a result, the breadboard allows us to conveniently attach circuit components together and quickly generate circuits. The front and inside of a breadboard is shown below. Do NOT remove your breadboard back like we did below. We dirtied our hands so you don't have to.

• Since we're just getting started, a graphical image of what the signal bridge circuit would look like on a breadboard is shown below.

• Grab/cut four small hookup wires. Use two of the wires to connect the two output wires to two electrical nodes in the breadboard. Wire strippers are on the cart in the corner! Wires are on spools on the other side of the lab!

  The wire cutters/strippers look like this:

  

  It has a sharp part (right near the hinge) that you can use to cut off a segment of wire. Then if you put the end of that wire into one of the holes near the top (the one labeled 24 AWG / 0.5mm is probably the right one), clamp down, and pull away, you can strip the plastic casing off the end of the wire

• Use two more wires to connect the Channel 1 oscilloscope probe to the function generator output through the breadboard.

• MAKE SURE that the "ground" (black) connection of from the function generator gets connected to the ground connection on the oscilloscope probe and the signal conductor from the function generator (red) gets connected to the actual signal conductor on the oscilloscope.

Once your circuit is complete, let's generate a signal.

2.2) The Function Generator

The DSOX1204G scopes have an in-built function generator. This is an output that can generate various electrical signals which are often useful for probing the system being studied. Follow these steps to generate our first test signal:

• Turn on the waveform/function generator by pushing the Wave Gen button. This will bring up a menu.
• Under this menu, click the "Settings" button (the bottom one on the screen), then click the "Default Wave Gen" button (the bottom one again) You should get a message on screen saying that things have been reset.
• Click the "Wave Gen" button again to go back to the menu where you can choose the form of the signal you're generating.
• Set the output signal to be a sine wave with:
  • A 10 kHz frequency
  • An amplitude of 1Vpp (1 Volt peak-to-peak)
  • An offset of 0V

Click the Channel 1 button and then using the appropriate option key, turn BW Limit to be on. This will cut down some high frequency artifacts that are potentially confusing the scope.

2.3) Vertical and Horizontal Scales

Once you've created the appropriate signals, use the horizontal and vertical axis controls (the big knobs under the "Horizontal" and "Vertical" labels on the front of the oscilloscope) to adjust the scaling of what gets rendered. Have fun. Go wild. Zoom in and zoom out to your heart's desire! Wheeee!

As you're zooming in and out, the dimensions of the plot are shown in the top left (for the y axis) and the top-right (for the t axis). Take a second to find them for your signal.

We'll use those to zoom in so that we get a really good view of our signal. Zoom in so that you're seeing:

• 200 milliVolts per division on the vertical axis, and
• 50 microseconds per division on the horizontal axis.

Feel free to play around with the wave gen controls, too (adjusting frequency, amplitude, and offset and seeing how the wave changes), but before moving on to the next part, set them back to a 10kHz sine wave with 1V peak-to-peak and 0V offset.

Use those to verify that the measurement you're seeing matches what we we're expecting based on the settings we put into the wave generator.

2.4) Measuring the Signal

The signal we generated in the previous section was created by us and we're measuring it directly so we sort of already know what its quantitative attributes are. But what if we didn't? How can we determine stuff about the signal. This is where the oscilloscope's measurement abilities come into play.

We have a number of options available to us for measuring various properties of the signal:

• We can eyeball measurements using the scales on the scope; each little grey box on the display represents the value we chose by zooming in and out, so we can count boxes to get approximate measurements
• We can use cursors to measure things, too. If you click the "Cursors" button on the scope, it will bring up four little orange gridlines you can move around with the knob right next to that button; and the bottom of the screen shows quantitative measurements based on those cursors (for example \Delta X shows the difference between the two horizontal cursors (with units of time), and 1/\Delta X shows you the associated frequency, with units of 1/time).
• We can make the scope do some measurements for us by clicking the "Meas" button and then choosing the appropriate options from the menu that pops up; there's a whole bunch of different stuff the scope can measure!

You can play around with these things a bit for now, and then in the next section we'll ask you to make use of them.

3) A Second Signal

What we'd now like to do is generate a second signal to measure (in addition to our first signal). We're going to create a little circuit shown schematically below. Grab the two resistors from the resistor tray in lab (near the large wire spools) and build the circuit below using your breadboard (ask for help if needed!). When complete, attach probe 2 to the electrical node in between the 11 \text{k}\Omega and 22 \text{k}\Omega resistors.

We've labeled the drawers in the resistor cabinet with their values, but each resistor also has colored bands on it indicating the value of the resistance in question. Here is a nice diagram showing what the different bands and colors mean if you're interested. In this color scheme, the 11{\rm k}\Omega resistor will have a stripe pattern of brown-brown-orange (1-1-3, meaning 11\times 10^3\Omega). The gold band on the end indicates that this resistor is accurate to within 5% of that value (it won't be perfectly 11{\rm k}\Omega). The 22{\rm k}\Omega resistor has bands like red-red-orange (2-2-3, meaning 22\times 10^3\Omega).

Using a combination of auto-measurements and cursors, determine:

• The maximum voltage of the sinusoidal waveform on Channel 2, V_{max}
• The minimum voltage of the sinusoidal waveform on Channel 2, V_{min}
• The peak-to-peak amplitude of the sinusoidal waveform on Channel 2, V_{pp}
• The frequency of the sinusoidal waveform on Channel 2, f_{ch2}

There are a couple of things to be careful of here:

• The scaling of each channel is adjusted separately, so the two curves you're looking at won't necessarily be on the same scale; each channel's scale is shown near the top of the scope. If you want to see the relative heights of the waves clearly, make sure they're on the same scale.

  • This is also relevant for cursors; the measurements shown on the screen are tied to one channel's scaling; you can choose which channel you're measuring on the menu under 'Cursors' on the scope.

• There is some thickness to each of the curves we're looking at (subtle variations in our signal coming from noise that we want to ignore); if you're manually placing cursors, try to put them in the middle of that thickness instead of on the very top or bottom.

• When using the auto-measurement tools, the scope does those things graphically; in order to get an accurate measurement, zoom in on the signal as far as you can without it spilling off the edges of the screen.

Anyway, go ahead and make those measurements (and don't hesitate to ask for help if you get stuck), and then enter your results below:

Frequency of Ch 2 signal (in Hz)

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V_{max} of Ch 2 (in Volts):

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V_{min} of Ch 2 (in Volts):

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V_{PP} of Ch 2 (in Volts):

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Check Yourself 1:

Compare these measurements to what you see on Channel 1. Do these results make sense? How do they compare to the theoretical result from solving the circuit? What are some general conclusions that can be made?

Be prepared to discuss these results during your checkoff.

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4) Buttons

Now let's take a look at another component, a button (a.k.a. a momentary switch if you're feeling fancy). In a circuit schematic, we'll often draw a momentary switch like so:

While the button is pressed down, its terminals are shorted together; but when we let go of the button, its terminals are again disconnected. In a second, we're going to build the following circuit:

The red "+" and "-" tags there represent the place where we'll measure our output (which will change based on whether the button is pressed or not).

When the switch is open, what is the voltage across the button (between the two labeled locations), in Volts?

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When the switch is closed, what is the voltage across the button (between the two labeled locations), in Volts?

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4.1) Build It

Grab a button from the front of the room (feel free to choose any color you like), and another resistor (it doesn't super matter what value you use, but something in the 1k-100k range is probably reasonable). Leave your other circuit intact, but build this circuit alongside it, using the wave generator on the scope to provide a constant ("DC") 10 Volts and connecting Channel 1 on the scope where indicated. The buttons look like this:

Now try pushing down the button and holding it for a while. Does the voltage on channel 1 change as you expected it to? Zoom in enough so that you can see the voltage clearly changing.

Often, we're interested in shorter button presses, though. Try pressing the button down as quickly as possible (just a single, fast tap) and seeing if you can notice the voltage change. If you're lucky, you may see a brief little blip on the screen. But that blip goes away very quickly; if we wanted to measure anything about it, that would be difficult as things currently stand.

This idea, of wanting to capture and measure rare events, is one of the things that the oscilloscope is great for. In order to fix this, we can take advantage of a the scope's "trigger" capability. In a broad sense, a "trigger" is just some condition that the scope is looking for. We can configure the scope to keep an eye out for different kinds of events, and every time it sees that event, it will draw the signal to the screen for us. We can use this behavior to capture that quick button press and measure some things about it.

By default, the scopes are set with the triggers in "auto" mode. In that mode, the scope will redraw the signal every time the triggering event happens; but if a certain amount of time passes without seeing that kind of event, the screen will update anyway, regardless of what's going on. This mode is useful. For one, we can see how the signal is changing broadly.

Right now, using auto mode, set the vertical zoom level so that the voltages with the button pressed and released are both clearly visible, but fall at very different points on the screen (i.e., they should be far apart vertically but both visible). As you click the button, a few times, you should see the signal bouncing back and forth between those two levels.

Next, we're going to try to measure a really fast button press by setting the scope's trigger to be the kind of thing that happens when the button is pressed. Specifically, we're going to ask the scope to look out for the voltage crossing a particular value. We'll do this by setting the "trigger type" to "rising edge" (click the "Trigger" button in the bottom-right corner of the scope, then use the buttons near the screen to choose "Trigger Type" and use the Entry knob to adjust it to be "Rising Edge" if it isn't already set there).

This setting will cause the scope to trigger when the voltage crosses some threshold value, which we can adjust by turning the knob labeled "Trigger" near the bottom right of the scope (as you turn that knob, you'll see a little line marked T move up and down). Slide the threshold around until it is comfortably between the two voltages we're seeing from our button.

OK, so now our scope is looking for that event; so we should clearly see a little blip when we briefly press the button. But in "auto" mode, the scope doesn't hang on to that signal for us, so it's hard for us to get a good look. In order to make the scope hang on to that signal and leave it on the display, we're going to turn the trigger to "Normal" mode instead. In normal mode, the scope will only redraw the display when the triggering event happens. This leaves that event centered on the screen, at which point we can zoom in and out, make measurements, and all that good stuff.

To summarize:

• In Auto Mode: The oscilloscope will capture and render when the trigger event happens, but if after a certain period of time no new trigger events happen, the trigger will be "forced", meaning it will make another capture/render based on samples periodic in time
• In Normal Mode: The oscilloscope will ONLY capture and render when the trigger event happens. The trigger will never be forced.

Try this out now: set the trigger to normal mode by hitting the "Trigger" button and then using the buttons near the screen and the Entry knob to adjust the trigger type to "Normal". Try pressing the button again, with just a single fast button press: what happens?

Check Yourself 2:

With the trigger still in normal mode, and the threshold properly set, press the button a single time, letting it go as quickly as you can. We're going to try to see how brief of a button press we can make (quantitatively!).

Once you've done that, without using cursors or the built-in measurement tools, just using the grid lines: how long was the button held down for? You will have to notice the time per division spec, in the top left and right parts of the oscilloscope screen respectively. The text can be very small, so be careful here. Notice that there are 10 divisions across the screen (i.e. it is the large, not the small divisions that matter).

You may also need to zoom in and out to get a really good view of the button press; we want to be able to see the whole button press event on screen (i.e., seeing both when you first press it and when you let it go) to be able to reason about it.

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Check Yourself 3:

Try setting the "slope" to "Falling Edge" instead of "Rising Edge" in the trigger menu and then repeat your experiment. What is different about the display between those two cases? Can you figure out what each of those settings is looking for?

Set it back to rising edge afterwards.

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Checkoff 1:

Talk with a staff member and answer the following questions:

• What is an oscilloscope displaying?
• What are the three main ways to measure attributes of a signal using an oscilloscope?

Also be prepared to demonstrate and/or discuss the following:

• How you measured the four attributes of your mystery signal through the circuit (Ch2)

  • Any common patterns you noticed about how Channel 1's value is related to Channel 2
  • How these results compare to the result from solving the circuit analytically

• What is a trigger? What is the difference between Normal and Auto modes?

• How long does the button stay pressed for on a single rapid button press? How can you tell by looking at the scope?

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5) Interfacing With the Real World

One powerful feature of many kinds of circuit components is their ability to sense things about the physical world that humans are not capable of sensing; and oscilloscopes give us a way to visualize things that we couldn't sense on our own.

In the remainder of the lab, we'll experiment with a couple of devices whose parameters vary based on their physical environment. These kinds of components give us a means of transduction, of converting information from the physical world (about light, temperature, sound, force, etc) into electrical signals, which can then be visualized (or acted on) by other devices.

5.1) Photoresistor

The first new component we'll look at today is called a photoresistor. This is a special kind of resistor whose resistance depends on the amount of light falling on it.

The physical device (shown on the right below) has two terminals just like a regular resistor, but its shape is different and it has a squiggly line on it. Schematically, we will represent photoresistors using the glyph shown on the left below (a resistor with two little arrows pointing at it representing light).

     

We're going to use this component to build a little circuit to help us detect light. But before we do that, let's get to know this component a little better. In particular, let's start by measuring its resistance. Unfortunately, the oscilloscopes don't give us a way to measure resistance directly, but we do have a different device that will allow us to make that measurement directly: a multimeter. Every lab station should have one (let us know if not; we have spares!). They're little and green and they look like this:

They might be up on the shelf above your lab station if they're not right in front of you.

These meters are a little fancier than the orange ones we sent home with the pset; they can measure all the same things, but these green ones feature auto-scaling, so instead of manually choosing what precision we want to be measuring with, the meter will do that for us.

Turn on the multimeter and set it to measure resistance by turning the little dial to \Omega (Ohms). Then grab a photoresistor from the cart in the back corner of the room. If you clip the red clip from the multimeter to one terminal of your photoresistor and the black clip to the other, you should start seeing resistance values come up on screen. Let us know if you're having any trouble.

Once you have those connected up, you should see measurements start appearing. That measurement contains not only a number, but also units. Pay close attention to the units that are displayed at the top of the screen! If you see "{\rm M}\Omega", the value you're seeing is in megaOhms. If that were a "{\rm k}\Omega" instead, that would mean kiloOhms, and if it's just "\Omega", we're just seeing Ohms, and so on.

Answer the following questions, and be prepared to discuss during your checkoff:

• What is the resistance of the photoresistor in ambient lighting?
• What is the resistance if you cover the end of the photoresistor with your hand, blocking the light? (be careful not to let the leads touch each other!)
• What is the resistance if you shine a bright light on the photoresistor from close by?

Write those down.

Feel free to ask a staff member for help if you're unsure of the values you're seeing.

With those values in mind, let's build another circuit to create a voltage that depends on the brightness of the lights in the room. Take a look at the circuit below. Before building it, predict: as the amount of light goes up, will the voltage indicated below go up, down, or stay the same?

Now, grab yourself a new 1{\rm k}\Omega resistor from the cabinet (brown-black-red for 1-0-2, 10\times 10^2\Omega) and lay the circuit out on your board, setting up the wave generator on the scope to create a constant ("DC") value of 10 Volts by changing the Wave Gen "Waveform" selector to "DC" and the "Offset" to 10V.

We'll start by using the multimeter (not the scope yet) to measure the voltage. With your multimeter on, click the "DC V" button to measure a voltage, and then connect the alligator clips from the multimeter to the spots indicated in the drawing above (the red clip from the multimeter should go to the spot labeled CH1+ on the drawing above and the black clip should go to the spot labeled CH-). What is the voltage drop there, and how does it change as you shade the photoresistor or shine a light on it?

Now let's hook it up to the scope instead. Disconnect the multimeter probes (you can also turn off the multimeter now, it's done its job for the day) and replace them with your scope probes. Click the "Default Setup" button, then click the channel 1 button and make sure the "BW Limit" is turned on.

Try shading the photoresistor and shining a light on it again. How does the display change as you do that? Does this make sense given what you saw on the multimeter?

5.2) Something Unexpected

Probably, what you just saw looked like a straight line (representing a more-or-less constant voltage) that moved up and down as you varied the lighting conditions. But it turns out that there is also something else going on there! We can't really see it, but the scope is going to help us out.

Mostly what we're measuring here (unless you're shining your own light on the photoresistor) is light from the overhead fluorescent lights. To human eyes, this looks like it's constant; but it turns out that it's actually changing slightly (and not at random, but in a very regular and predictable way).

This is one place where circuits (and the scope) really shine. What we're going to do is take this (likely inperceptible) change in lighting conditions and turn it into an electrical signal, which the scope can help us visualize.

What that change in lighting conditions means for our circuit is that there will be a small oscillatory voltage added on top of the constant-looking voltage we were seeing before.

To find this signal, let's do the following:

• Firstly, the offset in the signal can change pretty dramatically as we're moving around, casting shadows, etc. We can make the scope ignore that and show us only the oscillatory parts of our signal by clicking the channel 1 button and changing "Coupling" (the top option) to "AC" instead of "DC". This will cause the scope to show only the oscillatory parts of our signal, ignoring any constant offset that might exist.

  When you make this change, you should see the line jump down to 0 on the display.

• Now, let's zoom in. Zoom in to around 5 milliVolts per div in the vertical dimension and 5 milliseconds per div in the horizontal dimension. You should start to see something that looks like a sine wave.

  Use the measurement tools and/or the cursors here. What is the frequency of this oscillation?

  Note again that this is not an artificial signal at all; what we're seeing there is really a measure of the tiny, almost-imperceptible flickering of the lights in the room !

5.3) Another Awesome Sensor

Let's also try using another cool circuit element whose properties change based on the physical environment in which it is placed: a microphone! The picture below shows the microphone we'll be using in today's lab from a couple of different angles. Note that it also has two terminals.

       

When connected in a particular configuration (like we'll do in just a second), this microphone will behave like a current source, setting its current to be some value proportional to the instantaneous air pressure (so that sound waves entering the microphone cause relatively strong, oscillating values of current).

Before building the circuit, set the wave gen output to be a constant ("DC") 10 volts, and click the "Default Setup" button on the scope.

Did you do it? And you clicked "Default Setup"?

Good.

Now for another circuit. Leave your photoresistor circuit in place (we'll need it during the checkoff), and build the following circuit on a different portion of the board. Note that the current source drawn below represents the microphone (and the current through it will change as we make sounds).

Either grab a 33{\rm k}\Omega resistor from the cabinet, or, for a little extra challenge, you can try making something that behaves like a 33{\rm k}\Omega resistor using the components you already have from earlier in the lab.

You may need to zoom in a bit using the scope, but once you've zoomed around properly, you should be able to see a noticeable change if you snap your fingers or clap your hands right next to the microphone. Make sure that you've got "BW Limit" set on the channel you're using to measure; otherwise it can be hard to see.

Leave the scope in Auto mode while doing that experimenting so that you know where to set the threshold for your trigger.

Then, use the triggers to "freeze" on one of those events by putting the trigger in Normal mode, setting it appropriately, and snapping/clapping. You should see a small blip stay put on the screen (until you do it again, at which point the graph should change). Again, no worries if you get stuck; we're happy to help!

You may also wish to try speaking or whistling or something like that and observing how the signal changes. It's kind of neat to whistle at a couple of different pitches and see the signal change. But mostly be prepared to discuss the clapping/triggering example during checkoff.

Checkoff 2:
Discuss the oscillscope and triggering with a staff member. Be prepared to do/answer the following questions:

• Demonstrate both of your circuits and the measurements you've made.
• What is a trigger? What are the different types of triggering?
• Why is a trigger necessary on an oscilloscope?
• Discuss the difference between Normal Mode and Auto Mode.

Be prepared to show the staff member your signal you measured.

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6) Remote Control

OK, now it's time to put your oscilloscope chops to the test by measuring another real-world signal.

We have a bunch of Roku remotes that someone was selling on eBay for super cheap. Foolish move, random eBay user! I guess you don't know what you've got til it's gone. But they're ours now, no take-backs, so let's experiment with them.

Grab yourself a remote control from the front of the room and make sure it has batteries in it (we probably have extras if not).

Remote controls like these ones work by blinking an infrared light in a predictable pattern. These pulses aren't visible to human eyes (evolution hasn't granted us infravision yet), but your TV or toaster or whatever you're controlling with it has sensors that can see the blinking, and, based on the pattern, determine what button you're pushing.

In this last part of the lab, we'll build a detector for the signals coming out of the remote control and be able to see what your TV sees. In fact, we'll not only measure these signals; we'll try to listen to them as well by turning that light-based signal into an auditory signal.

To do this, we'll need a few different components that we've not used so far. In particular, we'll use a phototransistor, a different kind of light-sensitive device. The phototransisitors we have on hand are particularly sensitive to infrared light, so they're a great fit for this application. When we maintain a sufficient voltage drop across the terminals of the phototransistor, it behaves a lot like a current source, where the current at any point in time is roughly proportional to the amount of infrared light falling on the phototransistor at that point in time.

So in our lumped-element model, this works a lot like the microphone from the last part, and we can build a very similar circuit for this application to what we used to measure the output of the microphone. But here instead of a fixed resistor up top, we'll use a speaker (which behaves something like a resistor electrically but makes sound).

After you have this built, try pointing the end of the remote control at the phototransistor and holding down a button. You should see something on the scope, and you should also hear a series of little squawks coming out of the speaker (which come from interpreting the light pattern as a sound). You probably need for the remote to be pretty close to the sensor in order to hear anything, and you should make sure you're pointing the remote straight down on the top of the phototransistor, not from the side.

Use what you've learned about triggering, zooming, and making measurements with the scope to answer the following questions, and be prepared to demonstrate how you calculated these things during your checkoff. Note that using the scope's built-in measurement tools probably won't work great to answer any of these questions; you should use your cursors instead.

You'll want to start pretty far zoomed out horizontally so that you can see multiple "beeps" on the screen at the same time, and then you can zoom in to see more detail to answer the questions.

How long is each little beep of sound? Enter your answer below as a single number, in milliseconds.

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How long is the pause between each beep? Enter your answer below as a single number, in milliseconds.

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Within each beep, we are hearing a particular frequency associated with the light turning on and off. Find that the most rapid changes within a single beep; the frequency that we hear will be that one. Find this frequency (in Hertz), and enter it below. As a hint, humans can generally perceive frequencies between around 20Hz and 20kHz, so any audible frequency would be in that range.

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There is another frequency even higher than that, what we call the 'modulation' frequency, if you look within each individual little pulse of light. If you're curious as to why the remote is designed to do that, we're happy to talk about it; but for now, let's measure that frequency. What is that frequency, in Hz?

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Additionally, be ready to talk about the following during your checkoff:

• The pulses sound the same regardless of what button you're pushing down, but we can use the scope to notice some subtle differences. What changes as you change what button you're pushing?
• How does the signal change as you change the distance between the remote and the phototransistor? What about the angle between them?

Checkoff 3:

Discuss the results of these experiments with a staff member, and demonstrate using the scope to make the measurements above.

After that, you're all set for this lab! Well, almost...

After your checkoff, clean up by:

• putting your button, photoresistor, microphone, phototransistor, speaker, remote control, and breadboard back up where you got them on the cart
• locking your wire stripper (by closing it down and sliding the mechanical lock lever into place) and putting it back on the cart as well
• making sure your multimeter is turned off by clicking the dial until it says "off"
• throwing away all constant resistors and clipped wires (please don't put the constant resistors back in the cabinet)
• hanging the scope probes back up on the hangers

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