Sensor:
A sensor is a device that receives and responds to a signal. Sensors are specific types of transducers that are designed to sense or detect changes in the environment and convert these changes into electrical signals.
Transducer:
A transducer is a device that converts one form of energy into another. They can convert physical quantities such as temperature, pressure, and light into electrical signals, or vice versa. In essence, transducers are capable of both sensing and actuating.
Actuator:
An actuator is a device that converts energy into motion. Therefore, it is a specific type of a transducer. They are used to control or manipulate systems by converting electrical signals into mechanical movement, such as turning a motor or opening a valve.