Explained
How a smartwatch works.
How a smartwatch works
A smartwatch packs a small computer, a display, radios and a set of health sensors into a sealed case on your wrist. The layout is a stack. The crystal and display are on top, then the electronics and battery, and the sensor window sits on the back against your skin. The crown and side button poke through the case wall with seals around their shafts. This model is generic and unbranded.
Everything is designed around a tiny battery. The display can drop to a very low refresh rate, the processor sleeps between tasks, and sensors take readings in short bursts rather than all the time. That is how a watch can last a day or more on a battery that holds a small fraction of a phone's energy.
Reading your heartbeat with light
The green lights on the back are the heart-rate sensor. The method is called photoplethysmography, or PPG. Blood is red because it reflects red light and absorbs green light. Each time your heart beats, a pulse of blood swells the tiny vessels in your wrist, so a little more green light is absorbed and a little less comes back to the photodiodes. Counting those dips gives your pulse.
Movement creates noise in the signal, so the watch also uses its accelerometer to tell a heartbeat from the jolt of a footstep. Many watches add infrared light for overnight readings and electrodes for other measurements, but the basic idea stays the same: measure light, find the rhythm.
Crown, haptics and the band
The digital crown is a rotary encoder. As you turn it, a sensor counts tiny steps of rotation and the software turns them into scrolling or zooming, so your finger never covers the small screen. The haptic motor is a linear actuator that shakes a small weight to tap your wrist, which is subtle enough to feel without anyone else noticing.
The band slides into rails cut into the case. It matters more than it looks: a band that is too loose lets light leak in around the sensor and makes heart readings less reliable. Explode the model to see every part, or open Play to remove the layers in the order a repair would follow.
Parts glossary
- Cover crystal
- Strengthened glass or sapphire protects the screen. Sapphire is very hard, so it resists scratches from everyday grit. Fun fact: Sapphire scores 9 on the Mohs hardness scale. Only diamond, at 10, is harder among common reference minerals.
- Display
- A small OLED panel with a touch layer. Because each pixel makes its own light, the watch can keep a dim face on using very little power.
- Case
- The metal case is the frame for everything else and helps seal the watch against sweat and water.
- Digital crown
- The crown is a rotary encoder. It senses how far you turn it, so you can scroll without covering the screen.
- Logic module
- The processor, memory, storage and radios are packaged into one small sealed module to save space.
- Battery
- A small lithium-ion cell. Power is so tight that the software carefully schedules every sensor reading.
- Haptic motor
- A tiny linear motor taps your wrist for notifications.
- Speaker and microphone
- Small openings in the case let sound in and out while membranes keep water out.
- Heart-rate sensor
- Green LEDs shine into your wrist and photodiodes measure how much light comes back. That is photoplethysmography. Fun fact: Blood absorbs green light, so each heartbeat's pulse of blood briefly reduces the light that bounces back.
- Band
- The strap slides into rails in the case. A snug fit keeps the sensor against your skin for better readings.
