Transformer: Converts one electrical quaintly to another electrical quantity. Usually to convert the potential difference of an alternating current to another potential difference while maintaining most of the power. However transformers can be used for Impedance matching and even current conversion as well. For example a transformer can be used to connect many 8 ohm loud speakers to one 8 ohm amplifier without affecting the load on the amplifier.

Transformers only work with AC however the term "DC transformer" is also some times used to describe a DC-DC converter module.

Transducer: converts a physical quantity into an electrical quantity (in Electrical Engineering). This means that anything in the physical nature besides a few things such as mass can be converted into some measurable electrical quaintly in the form of electrical potential, impedance, capacitance, inductance time or current. Mass is usually determined by means of weighing. The weight is measured and then converted in to an equivalent mass by means of electronic calibration. Mass for example is the quantity of matter, A strain gauge or load cell transducer cannot measure the quantity of matter directly. Therefore they measure the force of gravity that exist between the earth and that object under test. Thus we measure the weight and not the mass. Therefore not all physical quantities can practically be converted to an electrical quantity. Displacement can be measured with an LVDT while distance is not measured with a transducer but in the case of laser distance measurement or movement detectors the transducer element is a optical transducer such as a photo diode or photo transistor or CCD. Not all quantities that is electronically measurable is done by means of a transducer to sense that quantity directly but indirect methods are often used. Like speed or distance can also be done by phase or frequency analysis of radio waves. In the case of speed, the Doppler radar is not a transducer. But the RF antenna is, because it converts electromagnetic waves to electric fields that can be received by the instrument. The transducer is purely the element responsible for converting the one physical quantity to another.

One must be careful for not confusion transducer with instrument. There are not much hard and fast rules to make sure about that definition. For example: A load cell is a transducer. But we get digital load cells with complicated digital circuits inside them with calibration and all, it is still a transducer regardless. We get a pressure gauge with a user interface such as a display built in and it is still a transducer. But a weighing scale with a user interface is an instrument and not a transducer, the load cell or strain gauge inside is the transducer. A Doppler camera for measuring traffic speed is not a transducer, it is an instrument but the antenna it uses is a transducer. If there is a hard and fast rule one could define it with it would be something like this: the smallest component that the instrument can be broken down to (disassembled to) that can convert a physical quantity to and electrical quantity is a transducer.

Some other uncertainties: Some definitions may consider a transducer as device that convert one form of physical energy to another, however in electrical engineering we often do not require energy to be converted and we only need a transducer to purely convert any kind of quantity into a electrically measurable characteristic such as impedance, capacitance, inductance that might have nothing to do with energy conversion.

Here are some examples of transducers:

Antenna - converts electromagnetic waves into electric current and vice versa

Cathode ray tube (CRT) - converts electrical signals into visual form

Fluorescent lamp, light bulb - converts electrical power into visible light

Magnetic cartridge - converts motion into electrical form

Photodetector or photoresistor or light dependent resistor (LDR) - converts changes in light levels into resistance changes

Tape head - converts changing magnetic fields into electrical form

Hall effect sensor - converts a magnetic field level into electrical form only

Disk read-and-write head - converts electrical digital signal into magnetic fields .

pH probes

Electro-galvanic fuel cell

Hydrogen sensor

Electromechanical (electromechanical output devices are generically called actuators):

Electroactive polymers

Galvanometer

Microelectromechanical systems

Rotary motor, linear motor

Vibration powered generator

Potentiometer when used for measuring position

Load cell - converts force to mV/V electrical signal using strain gauge

Accelerometer

Strain gauge

String potentiometer

Air flow sensor

Tactile sensor

Loudspeaker, earphone - converts electrical signals into sound (amplified signal → magnetic field → motion → air pressure)

Microphone - converts sound into an electrical signal (air pressure → motion of conductor/coil → magnetic field → signal)

Pickup (music technology) - converts motion of metal strings into an electrical signal (magnetism → electricity (signal))

Tactile transducer - converts electrical signal into vibration ( signal → vibration)

Piezoelectric crystal - converts solid-state electrical modulations into an electrical signal (vibration → electrical current → signal)

Geophone - converts a ground movement (displacement) into voltage (vibrations → motion of conductor/coil → magnetic field → signal)

Gramophone pickup - (air pressure → motion → magnetic field → signal)

Hydrophone - converts changes in water pressure into an electrical form

Sonar transponder (water pressure → motion of conductor/coil → magnetic field → signal)

Laser diode, light-emitting diode - converts electrical power into forms of light

Photodiode, photoresistor, phototransistor, photomultiplier tube - converts changing light levels into electrical form

Electrometer

Resistance temperature detector (RTD)

Thermocouple

Peltier cooler

Thermistor (includes PTC resistor and NTC resistor)

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