TRANSDUCERSDevice which provides a usable output in response to a specified measurand. Here, the output is defined as an electrical quantity and measurand as a physical quantity, property or condition which is measured.SensorsA sensor is a device that detects and responds to some type of input from the physical environment. The specific input could be light, heat, motion, moisture, pressure, or any one of a great number of other environmental phenomena. The output is generally a signal that is converted to human-readable display at the sensor location or transmitted electronically over a network for reading or further processing.Here are a few examples of the many different types of sensors:In a mercury-based glass thermometer, the input is temperature. The liquid contained expands and contracts in response, causing the level to be higher or lower on the marked gauge, which is human-readable.

ActuatorsConverts a signal(usually electrical) to some action, usually mechanical.

Difference between Sensors and ActuatorsA Sensor can sense in any form(usually electronic) i.e due to some mechanical change, it can react in electrical form. Thus there is a conversion, similar to that of a transducer. A classic example would be a thermocouple. Or a pressure sensor which might detect pressure and convert it into electric current (3-15psi to 4-20ma)

A Transducer is more than a sensor. It consists of a sensor/actuator along with signal conditioning circuits.A signal conditioning circuit, by the name is a circuit which conditions the signal so that it is strong enough for further processing. A system might contain many stages before the signal finally reaches its destination to derive meaningful information. The purpose of a sensor is to detect and measure, and whether its efficiency is 5% or 0.1% is almost immaterial, provided the figure is known. A transducer by contrast, is intended to convert energy, and its efficiency is important, though in some cases it may not be high.Active and Passive TransducersModulating transducer requires an auxiliary energy source Strain gauge, thermal resistor, liquid-crystal-display. Self-generating transducer requires no auxiliary energy source Solar cell, thermocouple, piezoelectric element