Transducer: A device that converts variations in a physical quantity, such as pressure or  brightness, into an electrical signal, or vice versa.

Sensor: A transducer that converts variations in a physical quantity, such as pressure or  brightness, into an electrical signal, which is then usually fed into the input of a computer controlled system.

• Sensors can be further classified as simple or advanced sensors, or as analog or digital sensors.

Simple sensor: A sensor that produces a simple output electrical signal such as an analog voltage or a logic HIGH or LOW value.

• A simple sensor usually consists of a simple variable resistance transducer or a simple mechanical transducer.

Advanced sensor: A sensor that usually has a micro-controller or an advanced electrical circuit embedded in the sensor itself to control its operation.

• In advanced sensor the builtin micro-controller or circuit communicates the measurements to another external micro-controller by using a communication protocol.
• Common communication protocols used by advanced sensors are:
• PWM: Pulse Width Modulation
• Binary Code: Series of binary values (e.g. 10011110 01100001)
• UART: Universal Asynchronous Receive Transmit protocol
• SPI: Serial Peripheral Interface protocol
• I2C (IIC): Inter Integrated Circuit protocol
• USB: Universal Serial Bus protocol
• It is important to note that all of the above communication protocols use serial transmission (bits are transmitted one by one)