• The term sensor is used for an input device that provides a usableoutput in response to a specified physical input.• For example, a thermocouple is a sensor that converts a temperaturedifference into an electrical output.• The term transducer is generally used to refer to a device that convertsa signal from one form to a different physical form.• Thus sensors are often transducers, but also other devices can betransducers, such as a motor that converts an electrical input intorotation.• Sensors that give digital or discrete, that is, on/off, outputs can beeasily connected to the input ports of PLCs.• An analog sensor gives an output proportional to the measuredvariable.