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Switches, regardless of their number of contacts and throw positions, can be controlled in one of two ways. The first and most obvious is manually, whereby a person or operator physically moves the operating handle of the switch, changing the state of its contacts. The second category is to automatically control the switch via some sort of process variable.

Many processes require automatic switching, independent of human operators for safety, accuracy, and efficiency. For instance, motors are often controlled by what are known as pilot devices that sense external conditions and provide two-wire control.

There are nearly limitless process variables which can be monitored by pilot devices, such as liquid level, proximity, temperature, and pressure.

All pilot devices share the same two-step process:

1. Sense external input.
2. Change the state of electrical contacts.

This text will focus mainly on the second of these two stages, as the connection of electrical contacts into our control circuits is where we will see the most utility for our pilot devices. Indeed an entire textbook could be written about the many types and configurations of various pilot devices, but this is beyond the scope of this book. Instead, we will examine simple control devices that respond to the condition of a process variable such as temperature, pressure, liquid level, flow, position or proximity by activating a set of contacts.

Float Switches

Float switches, also known as liquid-level switches, are pilot devices used primarily in pump-motor applications to respond to changes in liquid level. There are many different types of float switches, but the simplest mechanical arrangement is a pivoted arm that has the electrical contacts fastened at one end and a rod connected to a float at the other. As the water level rises or falls, the float moves with the water level and the contacts change their state accordingly.

For typical tank-filling services, float switches close their electrical contacts at a low-liquid level and open at a high level, while for sump pump purposes, the float switch closes the circuit at a high-liquid level and opens the circuit at a low-liquid level.

Temperature Switches

Temperature switches are used to provide automatic control in heating and cooling systems. Simple thermostats often use a coiled bimetallic element to activate a switch contact. This is sufficient for the simple heating and cooling of rooms.

For industrial processes that require monitoring of material temperature, a remote-bulb temperature switch is often used. The bulb is placed inside the medium to be measured and is linked via a capillary tube to a bellows. As the temperature of the fluid in the bulb rises, its volume expands and causes an internal pressure change. This change in pressure results in the movement of the bellows, activating a set of switch contacts.

Pressure Switches

A pressure switch is a device that is used to sense a rise or fall in pressure and is usually pipe-connected to the vessel containing the medium (such as water, steam, air, or gas) that operates the switch. Pressure switches are commonly used as safety devices to protect system equipment from either dangerously high pressure or low pressure.

When dealing with pressure controls, the term “cut in” refers to the pressure of the system when the switch closes, and this normally applies to decreasing pressure. The term “cut out” refers to the pressure of the system when the switch opens and normally applies to increasing pressure.

While the range of a pressure switch describes the overall pressure over which the switch can operate, the sensitivity of its contacts can usually be adjusted and the differential describes the difference between the cut-in and the cut-out pressure of the switch.

Flow Switches

A flow switch is activated by the movement of either liquid or air through a pipe or duct. For example, to ensure that heating elements are not energized if air or water is not flowing across heat exchangers, a flow switch might be utilized.

A simple type of flow switch uses a pivoted arm with contacts at one end and a paddle on the other, which is inserted into a pipe so that the flow of fluid activates a set of electrical contacts.

Mechanical-Limit Switches

Mechanical-limit switches are commonly used to prevent over-travel in elevators and along conveyor belts. The operation of a limit switch relies on an external force striking an operating arm or lever, which in turn activates a set of control contacts. Once the force is removed, the operating arm returns to its normal position and the contacts reset.

Special types of limit switches can provide a maintained-contact action, whereby the contacts remain in their changed position even after the initial force is removed from the activation arm. These types of limit switches need to be reset between uses, usually by simply moving the operating mechanism in the opposite direction.

Limit switch contacts are frequently used in applications where normally open contacts are mechanically held closed, or normally closed contacts are mechanically held open. Control schematics usually display symbols to indicate the state of the switch contacts when the system is in the “off” condition. For example, the symbol for a “normally open held closed” limit switch indicates that the contacts have been wired normally open, but when the circuit is put into its normal off state, part of the machine keeps the contact held closed.

Other types of pilot devices, such as those used in pressure and flow switches, can be configured in a similar way, but it is seen most often with mechanical-limit switches.

Proximity Switches

A proximity switch is a type of limit switch that does not require physical contact in order to function but rather senses the presence of particular materials. There are three main styles of proximity switches, each relying on different physical phenomenon to operate.

Inductive-proximity switch

Inductive proximity switches detect the presence of both ferrous and non-ferrous metals (ferrous refers to materials containing iron). They rely on the creation and monitoring of an electromagnetic field. An example is traffic sensors embedded into the pavement that sense the presence of the metallic body of the car and control the signal action of the traffic lights.

Capacitive-proximity switch

Capacitive-proximity switches detect the presence of almost any material. They rely on the creation and detection of an electrostatic field. Examples include the interactive touch screen of almost every modern electronic device, including smart phones and tablets.

Both inductive and capacitive types of proximity sensors use an oscillator circuit to generate a sensing field and series of solid-state devices to sense any change in the field, which in turn activates a set of external contacts.

Photoelectric switch

Photoelectric switches are proximity switches which emit and detect a beam of light from an infrared LED. If the beam is interrupted or interfered with, this is sensed by the receiver and a signal is sent to another series of solid-state devices, which in turn then operate a set of contacts. Examples include the sensors on automatic doors, including elevator doors, as well as touchless faucets that activate whenever a person places their hands in the sink to be washed.