A transducer (also called a sensor, pickup or pickoff) is a device which converts variations in some physical quantity into electrical variations.

It can also include a mechanical coupling designed to amplify the variation before it is applied to the electrical element.

Transducers can be divided into two categories, modulating and generating.

modulating transducers alter some electrical property (for example, resistance or capacitance) to convey the data (variation in the physical quantity being measured) whereas generating transducers produce a voltage proportional to the magnitude of the quantity (or in some cases, proportional to a CHANGE in the quantity)

The signal from generating transducers must be fed into an electrical amplifier before it can be used, since it is usually a very small signal of only a few millivolts (in some cases, microvolts).

Modulating transducers require a stable reference voltage or frequency, and are connected in an electrical circuit such that they alter the voltage, frequency or phase of the reference to reflect the variations in the physical quantity... the resulting signal is then amplified and used.

Whenever possible, a transducer is built into the device under consideration as an integral part of the assembly in order to avoid loading, dragging or otherwise altering the characteristics of the quantity being measured.

Transducers may be used to obtain:

Pressure (in solids, liquids and gases)
Flow rate (in liquids and gases)
Levels (of liquids in tanks)

other examples include:
position, speed, velocity, acceleration or vibration of parts within a machine (or of entire submarines, ships, aircraft or spacecraft)

Transducers may also be divided into Analog or Digital, depending on whether they provide a continuously varying signal (Analog) or only two possible states (Digital)

Examples would be an Oil Pressure Guage that continuously reads the actual pressure (analog) or an Oil Pressure Warning Light (digital) that illuminates when the pressure drops to some threshold value.


Generating types of transducers include photoelectric, thermoelectric and piezoelectric (electricity generated from external stress applied to a crystal)

Modulating types of transducers include variable-resistance (thermistors for temperature, photocells or light-dependent resistors and potentiometers actuated by all sorts of mechanical methods), variable capacitance (varacter diodes, proximity switches) or variable inductance (moving a core into a coil to increase its inductance)

© copyright 2002 Stephen E. Mendes, Barbados.

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