How a Smartphone Works: Touch, Battery, Sensors, 5G

Inside the slab

Imagine lifting the glass off a phone. Under the touch display lies a thin sandwich of layers. The battery takes the most room, often half the thickness. The main board carries the system-on-chip, which holds the processor, graphics and modem, plus the memory. Around them sit several cameras, a speaker, a microphone, a vibration motor and antennas hidden along the metal edge. The animation pulls the phone apart so that every part floats at its own height. The first handheld mobile call was made on 3 April 1973 by Martin Cooper of Motorola, with a phone that weighed over a kilogram.

Touch is electricity

Under the glass lies a grid of rows and columns made of a transparent conductor, usually indium tin oxide. Every crossing stores a little electric charge, like a tiny capacitor. Skin conducts electricity, so a fingertip near the glass steals a bit of that charge and the capacitance at the crossing drops. A controller scans the whole grid between 60 and 240 times a second and notes which crossings changed. That is why the display answers to a bare finger but ignores a wooden pencil, and why ordinary gloves do not work.

Where exactly is the finger?

Several crossings change at once, the strongest under the middle of the fingertip. The controller compares the values of neighbouring crossings and works out the centre of the touch to a precision far finer than the grid spacing, even though a fingertip is about 8 to 10 mm wide. Touch two fingers and two separate patches appear, which is how pinch to zoom works. Phones built around such multi-touch displays reached the mass market in 2007. In the animation a fingertip lands on the glass and the crossings beneath it glow.

Ions on a round trip

Inside a lithium-ion cell two electrodes face each other: graphite on the negative side and a lithium metal oxide on the positive side, separated by a liquid electrolyte. Charging pushes lithium ions into the layers of graphite, like cars into a multi-storey car park. While you use the phone they drive back, and the electrons that cannot cross the electrolyte run through your phone instead. Stanley Whittingham, John Goodenough and Akira Yoshino built the key steps between the 1970s and 1985, and in 2019 they shared the Nobel Prize in Chemistry.

How long will it last?

A battery label says 4,500 mAh at 3.85 V. The energy is 4.5 Ah × 3.85 V ≈ 17.3 Wh. If the phone draws 1.5 W on average, it runs for 17.3 / 1.5 ≈ 11.5 hours. A bright display, a game or a weak signal raises the power, while sleeping lowers it to a fraction of a watt. Heat harms lithium-ion cells more than anything else, so a phone should not be left in a hot car. Lithium-ion cells were first sold in 1991, and no everyday battery has yet replaced them.

Which way is down?

An accelerometer is a microscopic mass on silicon springs etched into a chip. When the phone moves or tilts, the mass shifts and a capacitance changes. At rest it feels gravity: lying flat, the z axis reads about 9.81 m/s², which is 1 g. Tilt the phone by 30 degrees and z falls to 9.81 × cos 30° ≈ 8.5, while x rises to 9.81 × sin 30° ≈ 4.9. From these numbers the phone knows the tilt and turns the picture. A gyroscope adds how fast you rotate, and a magnetometer feels Earth's field, about 25 to 65 microtesla, and works as a compass. Tilt the phone in the sandbox and watch the readings.

A word with the tower

A mobile network is a honeycomb of cells. A tower covers a few hundred metres in a city and several kilometres in open country. Phone and tower exchange radio waves, the same family as light but with much longer waves: from roughly 700 MHz to a few GHz for 4G and most 5G, and above 24 GHz for short-range 5G. Radio travels at the speed of light, so 3 km takes 3 / 300,000 s = 10 microseconds. Downloading 1 GB, which is 8 gigabits, at 200 Mbit/s takes 40 seconds. As you move, the phone is handed from tower to tower without dropping the call.

The smartphone