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### Introduction to Switch

A switch may be a device that’s designed to interrupt the present flow during a circuit, in other words, it can make or break a circuit. Every electrical and electronics application uses a minimum of one switch to perform the ON and OFF operation of the device.

Though it’s going to seem strange to hide the elementary topic of electrical switches at such a late stage during this book series, I do so because the chapters that follow explore an older realm of digital technology supported mechanical switch contacts instead of solid-state gate circuits, and a radical understanding of switch types is important for the undertaking. Learning the function of switch-based circuits at an equivalent time that you simply study solid-state logic gates makes both topics easier to understand, and sets the stage for an enhanced learning experience in Boolean logic, the mathematics behind digital logic circuits.

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### 01. Mechanical Switches

Mechanical switches are often classified into differing types supported several factors like method of actuation (manual, limit, and process switches), number of contacts (single contact and multi-contact switches), number of poles and throws (SPST, DPDT, SPDT, etc.), operation and construction (push button, toggle, rotary, joystick, etc), supported state (momentary and locked switches), etc.

Based on the number of poles and throws, switches are classified into the subsequent types. The pole represents the number of individual power circuits that will be switched. Most of the switches are designed to possess one, two or three poles and are designated as single pole, double pole and triple pole.

The number of throws represents the number of states to which current can undergo the switch. Most of the switches are designed to possess either one or two throws which are designated as single throw and double throw switches.

• ### Single Pole Single Throw Switch (SPST)

01. This is the basic ON and OFF switch consisting of one input contact and one output contact.

02. It switches a single circuit and it can either make (ON) or break (OFF) the load.

03. The contacts of SPST can be either normally open or normally closed configurations.

• ### Single Pole Double Throw Switch (SPDT)

01. This means it consists of two ON positions and one OFF position.

02. This switch has three terminals, one is input contact and the remaining two are output contacts.

03. The contact which is connected to the input by default is referred to as normally closed contact and contact which will be connected during ON operation is normally open contact.

04. In most of the circuits, these switches are used as changeover to connect the input between two choices of outputs.

• ### Double Pole Double Throw Switch (DPDT)

01. Two input contacts are connected to the one set of output contacts in one position and in another position, input contacts are connected to the other set of output contacts.

02. It behaves like a two separate SPDT configuration, operating at the same time.

03. It has six terminals, two are input contacts and the remaining four are the output contacts.

04. This is a dual ON/OFF switch consisting of two ON positions.

• ### Push Button Switch

01. Double-acting push buttons are generally used for controlling two electrical circuits.

02. It is a momentary contact switch that makes or breaks connection as long as pressure is applied (or when the button is pushed).

03. Push buttons are majorly classified into normally open, normally closed and double-acting push buttons as shown in the above figure.

04. The internal spring mechanism operates these two states (pressed and released) of a push button.

05. Generally, this pressure is supplied by a button pressed by someone’s finger.

06. This button returns its normal position, once the pressure is removed.

07. It consists of stationary and movable contacts, of which stationary contacts are connected in series with the circuit to be switched while movable contacts are attached with a push button.

• ### Toggle Switch

01. These are available in different ratings, sizes, and styles and are used for different types of applications. The ON condition can be any of their level positions, however, by convention, the downward is the closed or ON position

02. Most of these switches come with two or more lever positions which are in the versions of SPDT, SPST, DPST and DPDT switch. These are used for switching high currents (as high as 10 A) and can also be used for switching small currents.

03. A toggle switch is manually actuated (or pushed up or down) by a mechanical handle, lever or rocking mechanism. These are commonly used as light control switches.

• ### Limit Switch

01. The control schemes of a limit switch are shown in the above figure, in which four varieties of limit switches are presented.

02. These switches consist of a bumper type of arm actuated by an object. When this bumper arm is actuated, it causes the switch contacts to change position.

03. Some switches are operated by the presence of an object or by the absence of objects or by the motion of the machine instead of human hand operation. These switches are called as limit switches.

### Switches Applications

• A switch is an electrical device, usually electromechanical, used to control continuity between two points.
• Hand switches are actuated by human touch.
• Limit switches are actuated by machine motion.
• Processswitches are actuated by changes in some physical process (temperature, level, flow, etc.).