Question

what is the difference between a sensor and a transducer?

Answer 1

In the general case, a sensor is the complete assembly required to detect and communicate a particular event, while a transducer is the element within that assembly which accomplishes only the detection of the event. At the risk of getting too abstract, a transducer converts an input to an output that can be exploited to accomplish the sensing mission.

For example, a pressure sensor may use a diaphragm and/or strain gage to "detect" pressure differential across the diaphragm, but he complete sensor will additionally consist of a display element and the electronics required to energize and condition the output of the detecting transducer, as well as the elements required to house and drive the display feature.

A compass is a simple sensor of magnetic north, wherein the magnetic element in the compass is the transducer or "north detector" and the needle, housing and compass face comprise the remainder of the "north sensor".

In special cases, a sensor and a transducer can be the same. For example, a bi-metallic spring element can both detect temperature change, and may well be the entire sensor if a pointer is attached to the bi-metalic spring.