A look inside the smartphone

This exercise is a part of Educator Guide: Smartphones Overshare / View Guide

Directions: After students have had a chance to review the article “Smartphones overshare,” lead a classroom discussion based on the questions that follow.

Discussion questions:

1. How does a smartphone’s liquid-crystal display (LCD) work?

A liquid-crystal display (LCD) is divided into pixels, and each pixel is divided into a red, green and blue subpixel. Each subpixel has liquid crystal sandwiched between two optical polarizers, such that light must be vertically polarized to pass through one polarizer and horizontally polarized to pass through the other polarizer. Normally these polarizers together would block all light that tries to pass through both of them. Liquid crystals contain long molecules whose orientations change in response to electric fields, and the molecules can then change the direction of polarization of light. As an electric field is applied to the subpixel, the liquid crystal molecules in that subpixel change their orientation based on the amount of voltage applied. The intensity of light passing through both polarizers is thus controlled by the applied voltage, so that the mixture of subpixel intensities combine to create a specific color in a pixel at a particular point in time. Light is provided by light emitting diodes (LEDs) behind the LCD screen. For color images, different pixels are illuminated by LEDs of different colors, or by white LEDs with different color filters.

2. How does a smartphone’s touch-sensitive screen work?

Smartphone touch screens usually use capacitance, or the ability to store an electrical charge, to sense where they are being touched. Your fingers are moderately electrically conductive. When they are close to the screen, they act like one plate of an electrical capacitor; the other plate is an array of metal behind the screen. Moving your fingers closer to or farther from different areas of the screen affects the electrical capacitance of different sectors of the metal array behind the screen, and the capacitance changes are measured by connected circuits. Instead of your finger, you can also use an electrically conductive stylus. Electrical insulators (like wooden sticks) do not work.

3. How does a smartphone’s digital camera work?

Digital cameras use one or more lenses to focus light onto a charge-coupled device (CCD). A CCD is essentially a two-dimensional array of light-sensitive transistors, which are individual pixels of the detected image. The more light a given transistor or pixel receives, the larger electrical signal it produces. Filters of different colors pass light of different wavelengths on to different pixels, so that color images can be reconstructed.

4. How does a smartphone’s microphone work?

There are several types of microphones, but one of the most common types is a condenser microphone. Speaking into the microphone vibrates a thin metal plate, which is parallel to and very close to another metal plate. The two metal plates act like a capacitor. As the first plate vibrates, the electrical capacitance changes, and those changes can be measured by a circuit connected to both plates.

5. How does a smartphone’s fingerprint scanner work?

Different people have different fingerprint patterns. A smartphone fingerprint scanner stores the image of its owner’s fingerprint, then uses pattern recognition software to determine if it sees the same person’s fingerprint and should unlock, or if it sees someone else’s fingerprint and should remain locked.

6. How does a smartphone’s proximity sensor work?

An infrared light emitting diode (LED) sends out infrared light that the human eye cannot see. An infrared light detector nearby on the phone measures how much of that infrared light gets bounced back to the phone. The closer an object (such as your face) is to the phone, the more infrared light gets bounced back. That is used to switch off the screen when you hold the phone up to your face, or when the phone is face down on a surface or inside a pocket or bag.

7. How does a smartphone’s light level sensor work?

A light level sensor measures the amount of light in the environment around a smartphone, then adjusts the smartphone screen’s brightness based on that measurement. Light level sensors can be photoresistors, in which changing light levels changes the electrical resistance of the sensors, and therefore, the current flow through the transistor.

8. How does a smartphone’s GPS navigation work?

The Global Position System (GPS) is a swarm of more than 30 satellites orbiting Earth. Each satellite is constantly sending out signals about its current position and time. A GPS receiver can detect those signals, compare them to the current time and know how long (and thus how far) each satellite’s radio waves have traveled. If the GPS receiver can detect signals from at least four satellites (since there are three dimensions of space and one dimension of time) at the same time, it can pinpoint the receiver’s position.

9. What does an accelerometer measure and how does a smartphone’s accelerometer work?

An accelerometer measures how much an object is accelerating or decelerating. Smartphone accelerometers typically use piezoelectric sensors. Acceleration in a particular direction slightly compresses a piezoelectric crystal in that direction, generating a measurable voltage signal. The larger the acceleration, the greater the compression and the generated voltage. Keeping track of acceleration over time can indicate velocity, and keeping track of velocity over time can indicate position.

10. What does a gyroscope measure and how does a smartphone’s gyroscope work?

A gyroscope measures changes in the direction an object is pointed. Large mechanical gyroscopes are spinning wheels that want to keep spinning in the same direction. Smartphones use microelectromechanical system (MEMS) vibrating structure gyroscopes. In such a gyroscope, a small vibrating object (such as a piezoelectric crystal) wants to keep vibrating in the same direction, even if its surrounding case is rotated toward a new direction. As in an accelerometer, piezoelectric crystal signals can be detected electrically.

11. What does a barometer measure and how does a smartphone’s barometer work?

A barometer measures air pressure. Lower air pressure can indicate that the barometer is at a higher altitude. If the barometer remains in one place, falling pressure can indicate approaching storms, and rising pressure can indicate improving weather. Classroom barometers may have liquid levels that change with the air pressure. Smartphone barometers use MEMS piezoelectric crystals that are compressed by atmospheric pressure and produce a measurable voltage.

12. What does a magnetometer measure and how does a smartphone’s magnetometer work?

A magnetometer measures the strength and direction of a magnetic field. Like a needle compass that can be used to measure the Earth’s magnetic field and determine directions on a map, magnometers can also be used as metal detectors to sense nearby magnetic metals. Microelectronic magnetometers rely on Faraday’s law of induction, in which changes to the magnetic field passing through a wire loop induce measurable electric currents within the loop. Alternatively, microelectronic magnetometers can use the Lorentz force, in which a magnetic field exerts a measurable force on electric charges moving through a wire.

Discussion questions:

1. What useful things could be done with smartphone sensor data?

Monitoring health: exercise, diet, sleep, infectious diseases and noninfectious diseases. Monitoring cognitive performance: sleepiness, intoxication and markers for depression. Monitoring geographic patterns of infection, traffic patterns, breaking news stories, weather events and recent crime events. Assisting people with disabilities, language translation, cooking recipes and science experiments.

2. Based on information in the article, “Smartphones overshare,” what disreputable things could be done with smartphone sensor data?

Stealing money, financial information, passwords, contact lists, identities and files. Secretly activating video, audio, or other sensors to monitor or track people. Blackmailing people to obtain money or cooperation in exchange for encrypted/deleted files, not releasing recorded or stolen data, etc.

3. How could downloading harmful apps on smartphones be prevented while still allowing useful apps? List potential possible prevention techniques and include a potential downside to every technique (put the downside in parenthesis).

Screening each app individually before allowing it in a central app store (time consuming). Comparing app behavior to an expected baseline and notifying the user of deviations (correctly establishing a reliable baseline). Specifically requesting user permission for every new thing every app does (potentially time consuming and performance limiting). Limiting which data and the quality of data that apps can access (also limits performance).

Extension prompts:

4. What data is the graph “Smartphone acceleration varies by mode of transport” displaying (include units in your description)? What is the user likely doing with his or her phone while the data is collected? Explain your answer based on the data shown.

The graph is showing the accelerometer readings (in meters/second2) of a smartphone over a period of about 400 seconds, or 6 minutes and 40 seconds. During approximately the first 260 seconds, there are only small vibrations and intermittent accelerations (about 2 to 4 m/s2), which might suggest that the user is maintaining a fairly steady velocity by riding on the metro. During the last 140 seconds of the data shown, there are many relatively large accelerations (about 4 to 10 m/s2), which might suggest that the user is on foot and possibly using the phone.

5. Based on the graphs titled “Key tap tilts,” roughly how long does it take for a user to enter one letter? Physically, how would you explain the indicated differences among the three letters that are graphed?

On the order of approximately one second. If the user is holding the phone in landscape position (sideways) so the y-axis runs horizontally across the phone, rotation around it would show how much the phone is tilted toward or away from the user. Q is on the bottom of the keyboard, so the phone gets tilted strongly toward the user as it’s pressed, then tipped back. I is on the top of the keyboard, so the phone gets strongly tilted the other way, away from the user, then back. V is near the center of the keyboard, so there is much less toward-away tilt when V is pressed. In landscape position, the x-axis runs vertically across the phone, so rotation about it would show how much the phone is tilted to the left side or right side. V is near the right side of the keyboard, so the phone tilts strongly to the right and then back. Q and I are closer the center of the phone, so there is less tilt around the x-axis when those keys are pressed.

6. Based on the graph “As privacy increases, accuracy drops,” how does increasing privacy by distorting sensor data affect the accuracy of speech translation and the accuracy of speaker identification software?

Increasing privacy makes the accuracy of both speech translation and speaker identification decrease. However, it is harder to identify a speaker than to merely recognize words, even with no privacy and no distortion of sensor data. As privacy increases, the accuracy of speaker identification drops much more quickly than the accuracy of speech translation. Thus, with moderate privacy/distortion, speaker identification can be prevented, while speech translation is still reasonably accurate. 

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