A sensor is a device that can perceive and respond to changes in the external environment. It is usually composed of a sensitive element and a conversion element, which can convert the physical, chemical, or biological quantities sensed into standard signals that are easy to measure, transmit, and process.

The main types of sensors include:

1. Physical sensors: such as temperature sensors, pressure sensors, acceleration sensors, displacement sensors, etc., are used to detect changes in physical quantities.
2. Chemical sensors: such as gas sensors, humidity sensors, pH sensors, etc., are used to detect changes in chemical components or concentrations.
3. Biological sensors: such as enzyme sensors, immune sensors, DNA sensors, etc., are used to detect changes in biological molecules or biological processes.

Sensors have extensive applications in industrial automation, smart home, medical health, environmental monitoring, traffic control, etc. With the continuous advancement of technology, the types and functions of sensors are constantly expanding and upgrading, bringing great convenience to human life and work.