A device which when actuated by one form of energy, gives the output energy in the same form or another form, is called a transducer.
Those which are self generating type are called active transducers e.g. Thermocouples.
Those which are externally excited are called passive transducers e.g. resistance thermometers and thermistors.
Analog transducers convert the input physical quantity into analog output e.g. strain gauge whereas digital transducers convert physical input quantity into an electrical output in the form of pulses.
A straight question and its answer in simple words, it is a device converting one form of energy into other. We can also say that a device which converts physical energy to electrical signal is transducer. The other name for transducers is PICKUPS. They are extensively used in instrumentation field as instrumentation deals with measuring and controlling a number of variables like sound, flow, level, angle etc.
We generally prefer converting physical energy to electrical energy because of many reasons but most important reasons are −
Amplification of electrical signal is done easily. Now suppose you want to measure some mechanical form of energy and that is very small so detecting output will be difficult and if we convert them to electrical signals it can be easily amplified as per need.
The output when converted to electrical signals can easily be transmitted and processed.
Transducer can be divided in two parts. One is Sensing element and other is Transduction element. Sensing element is a detector which is responsible for sensing the element or it is the part which responds to phenomenon and Transduction element is used to transform the output of sensing element to electrical output. So it is dependent on sensing element, basically it is an electronic circuit.