Sensor chips are chips and small devices that measure something in the physical world, such as motion, pressure, temperature, light, sound or X-rays, and turn it into an electrical signal. They are the first link in every machine that reacts to the world: sense, convert, decide, act.
Many sensors are MEMS, short for microelectromechanical systems. A MEMS sensor is a tiny mechanical structure, such as a spring, a plate or a thin membrane, carved into silicon with the same methods used to make chips. An accelerometer measures acceleration: a tiny mass on springs moves slightly when the device speeds up, slows down or tilts, and the chip measures how far it moved. A gyroscope measures turning. A pressure sensor has a thin membrane that bends as pressure changes. A MEMS microphone has a membrane that moves with sound.
Other sensors measure light. An image sensor is a grid of millions of light-sensitive cells called pixels. Every digital camera is built around one. The image sensors in a car's cameras must see both a dark tunnel and bright sunlight in the same frame. X-ray detectors measure X-rays that pass through the body or through a suitcase, in hospital X-ray machines and in airport security scanners.
A sensor's signal is usually small and analog, changing smoothly. So an analog chip strengthens it and turns it into numbers, and a microcontroller decides what to do with them. Many newer sensors include the converter on the same chip and send out numbers directly.
Consider the airbag system of a hatchback in a frontal crash test at 64 km/h, the speed used in India's Bharat NCAP frontal test. Accelerometers sit in the airbag control unit in the middle of the car, and often in sensors near the front. In normal hard braking, a car slows at under 1 g, where 1 g is the acceleration of gravity, 9.8 metres per second every second. In the crash, the accelerometers measure more than 30 g within about 10 thousandths of a second. The control unit's microcontroller compares the readings with stored crash patterns and decides to fire the airbag about 15 to 20 thousandths of a second after impact. The airbag is fully inflated about 30 thousandths of a second later. The unit needs two separate sensors to agree before it fires. That way, a sharp jolt from a pothole cannot set off the airbag by itself.
Sensors are also in smaller things. A smart ring shines light into the finger and measures the reflection to work out the heart rate. A smart thermostat measures temperature and detects movement. A cooking robot weighs each ingredient with a load cell, a sensor that measures force. In factories, vibration sensors feed small AI models on TinyML chips. Motion sensors combined into one unit become an inertial measurement unit (IMU).