Control switches cont.

Speed switch
These switches sense the rotary speed of a shaft either by a centrifugal weight mechanism mounted
on the shaft, or by some kind of non-contact detection of shaft motion such as optical or magnetic.

Pressure switch
Gas or liquid pressure can be used to actuate a switch mechanism if that pressure is applied to
a piston, diaphragm, or bellows, which converts pressure to mechanical force.

Temperature switch
An inexpensive temperature-sensing mechanism is the "bimetallic strip:" a thin strip of two
metals, joined back-to-back, each metal having a different rate of thermal expansion. When the
strip heats or cools, differing rates of thermal expansion between the two metals causes it to bend.
The bending of the strip can then be used to actuate a switch contact mechanism. Other temperature
switches use a brass bulb filled with either a liquid or gas, with a tiny tube connecting the bulb to
a pressure-sensing switch. As the bulb is heated, the gas or liquid expands, generating a pressure
increase which then actuates the switch mechanism.

Liquid level switch
A floating object can be used to actuate a switch mechanism when the liquid level in an tank
rises past a certain point. If the liquid is electrically conductive, the liquid itself can be used as a
conductor to bridge between two metal probes inserted into the tank at the required depth. The
conductivity technique is usually implemented with a special design of relay triggered by a small
amount of current through the conductive liquid. In most cases it is impractical and dangerous to
switch the full load current of the circuit through a liquid.



Liquid flow switch
Inserted into a pipe, a flow switch will detect any gas or liquid °ow rate in excess of a certain
threshold, usually with a small paddle or vane which is pushed by the °ow. Other °ow switches are
constructed as differential pressure switches, measuring the pressure drop across a restriction built
into the pipe.
Another type of level switch, suitable for liquid or solid material detection, is the nuclear switch.
Composed of a radioactive source material and a radiation detector, the two are mounted across
the diameter of a storage vessel for either solid or liquid material. Any height of material beyond
the level of the source/detector arrangement will attenuate the strength of radiation reaching the
detector. This decrease in radiation at the detector can be used to trigger a relay mechanism to
provide a switch contact for measurement, alarm point, or even control of the vessel level.