There are hundreds of transducers, many of which are specified by the energy changes they perform. For example, a piezoelectric sensor contains a piezoelectric element that generates motion when subjected to a voltage or generates an electrical signal when subjected to a strain. The latter effect can be applied to accelerometers, piezoelectric vibration pickups or strain gauges. Electro-acoustic transducers can convert electrical signals into acoustic signals and vice versa. An example is a hydrophone, which responds to underwater acoustic waves and is useful in underwater acoustic detection. Photoelectric sensors respond to visible light to generate electrical energy. Electromagnetic sensors form a large group whose main categories are differential transformers, Hall-effect magnetic sensors, inductive sensors, inductive sensors, and saturation reactors. These work on electromagnetic principles.
Electrical sensors can be classified as active or passive. Active transducers generate current or voltage directly in response to a stimulus. An example is a thermocouple; here, if two connection points are at different temperatures, current will flow into a continuous circuit of two metals for power generation. Passive sensors will produce some changes in passive electricity, such as capacitance, resistance or inductance, which will stimulate. Passive sensors usually require additional power. A simple example of a passive sensor is a device that contains a section of wire and a moving contact that contacts the wire. The location of the contacts determines the effective length of the wire, which in turn determines the resistance through which the current flows. This is the simplest version of a so-called linear displacement sensor or linear potentiometer. For practical applications, such transducers use wire-wound, thin-film, or printed circuits to allow long resistances to be formed in relatively small devices. The longer the resistance, the greater the voltage drop across the device; therefore, the change in position is converted into an electrical signal.
The transducer can also produce pneumatic or hydraulic output. The pneumatic system communicates via compressed air. One example is a device in which motion is applied to a baffle through a pivot system that can be moved closer to or further from a nozzle that emits a stream of air. The amount of resistance generated by the baffle will affect the amount of back pressure behind the nozzle, thereby generating a pneumatic signal. The hydraulic system tends to be similar to the pneumatic system design except that the hydraulic system uses hydraulic (liquid) pressure instead of air pressure. The fluid principle applicable to the interaction between two fluid streams is also used to create a transducer.