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.

Control switches cont.

Selector switch
Selector switches are actuated with a rotary knob or lever of some sort to select one of two or
more positions. Like the toggle switch, selector switches can either rest in any of their positions or
contain spring-return mechanisms for momentary operation.

Lever actuator limit switch
These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever
pushed by the machine part. Often, the levers are tipped with a small roller bearing, preventing the
lever from being worn off by repeated contact with the machine part.

Proximity switches
Proximity switches sense the approach of a metallic machine part either by a magnetic or high-
frequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a
sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). More com-
plex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency
current, and electronically monitoring the magnitude of that current. If a metallic part (not nec-
essarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring
circuit. The symbol shown here for the proximity switch is of the electronic variety, as indicated by
the diamond-shaped box surrounding the switch. A non-electronic proximity switch would use the
same symbol as the lever-actuated limit switch.

Another form of proximity switch is the optical switch, comprised of a light source and photocell.
Machine position is detected by either the interruption or reflection of a light beam. Optical switches
are also useful in safety applications, where beams of light can be used to detect personnel entry
into a dangerous area.
In many industrial processes, it is necessary to monitor various physical quantities with switches.
Such switches can be used to sound alarms, indicating that a process variable has exceeded normal
parameters, or they can be used to shut down processes or equipment if those variables have reached
dangerous or destructive levels.

Control switches

An electrical switch is any device used to interrupt the flow of electrons in a circuit. Switches
are essentially binary devices: they are either completely on ("closed") or completely or ("open").
There are many different types of switches, and we will explore some of these types in this chapter.
Though it may seem strange to cover this elementary electrical topic at such a late stage in this
book series, I do so because the chapters that follow explore an older realm of digital technology based
on mechanical switch contacts rather than solid-state gate circuits, and a thorough understanding of
switch types is necessary for the undertaking. Learning the function of switch-based circuits at the
same time that you learn about solid-state logic gates makes both topics easier to grasp, and sets
the stage for an enhanced learning experience in Boolean algebra, the mathematics behind digital
logic circuits.

The simplest type of switch is one where two electrical conductors are brought in contact with
each other by the motion of an actuating mechanism. Other switches are more complex, containing
electronic circuits able to turn on or off depending on some physical stimulus (such as light or
magnetic field) sensed. In any case, the final output of any switch will be (at least) a pair of
wire-connection terminals that will either be connected together by the switch's internal contact
mechanism ("closed"), or not connected together ("open").

Any switch designed to be operated by a person is generally called a hand switch, and they are
manufactured in several varieties:

Toggle switch
Toggle switches are actuated by a lever angled in one of two or more positions. The common
light switch used in household wiring is an example of a toggle switch. Most toggle switches will
come to rest in any of their lever positions, while others have an internal spring mechanism returning
the lever to a certain normal position, allowing for what is called "momentary" operation.

Push button switch
Push button switches are two-position devices actuated with a button that is pressed and released.
Most push button switches have an internal spring mechanism returning the button to its "out," or
"unpressed," position, for momentary operation. Some push button switches will latch alternately on
or off with every push of the button. Other push button switches will stay in their "in," or "pressed,"
position until the button is pulled back out. This last type of push button switches usually have a
mushroom-shaped button for easy push-pull action.