A switch in a pneumatic or hydraulic system is usually a pressure switch. It provides on/off switching from a pneumatic or hydraulic source when a certain set pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall.
In pneumatic systems, pneumatic pressure switches can be used in switching off an electrically driven gas compressor when a set pressure is achieved in the reservoir or whenever there is no feed in the suction stage. In hydraulic systems, hydraulic pressure switches have various uses in automobiles, for example, switching on a warning light if engine oil pressure falls below a safe level, switching on brake lights automatically by detecting a rise in pressure in hydraulic brake pipes.