Electronic Parts

Discrete Components

Switches

A switch changes the path available to electric current. At its simplest, a switch can either complete a circuit so current can flow, or open the circuit so current stops. More elaborate switches can choose between several circuits, operate more than one circuit at once, or be controlled by motion, magnetism, electricity, or electronic signals.

The physical form of a switch and its electrical action are two different ideas. For example, a toggle switch and a rocker switch may look quite different but both can be wired as the same basic SPST or SPDT circuit.

A few common schematic symbols

These drawings are simplified examples. Schematic-symbol details can vary somewhat with the drawing standard and the type of equipment being documented.

How switches are described

A switch description often tells you more about its contacts than about the knob, lever, or button you touch. Terms such as SPST, SPDT, DPST, and DPDT describe the number of poles and throws. Other important terms include normally open (NO), normally closed (NC), momentary, and maintained or latching.

Learn about poles, throws, NO/NC contacts, and switch ratings.

Mechanical switches

Toggle, rocker, slide, pushbutton, rotary, and DIP switches are examples of switches intended to be operated directly by a person. Although their mechanisms and shapes differ, many are available with the same basic contact arrangements.

Mechanical switches: toggle, rocker, slide, pushbutton, rotary, and DIP.

Rotary and selector switches are especially useful for understanding the difference between a physical switch and its contact arrangement. One shaft can operate many electrically separate contact sections at the same time, from a simple two-pole selector to switches with a dozen or more ganged poles.

Sensing and automatically operated switches

Some switches are operated by something other than a person's finger. Limit and microswitches can detect mechanical position. Reed switches respond to a nearby magnet. Similar switching ideas are used in float, pressure, temperature, and other sensing devices.

Sensing switches: limit, microswitch, reed, and related devices.

Electronic and digital switching

A circuit does not always need moving contacts to switch current. Transistors, MOSFETs, and integrated-circuit analog switches can perform switching functions electronically. Digital logic also depends on devices being driven between distinct electrical states.

Electronic switching: transistors, MOSFETs, analog switches, and digital control.

Relays

A relay uses an electrical input to operate one or more switch contacts. Traditional relays use an electromagnet and moving contacts; reed relays and solid-state relays use different methods. Relays are especially useful when a low-power control circuit must operate another circuit or when electrical isolation is desired.

Relays: coils, contacts, reed relays, and solid-state relays.

A little switch history

Electrical switching is nearly as old as practical electrical communication. The telegraph key is a good early example: pressing the key repeatedly made and broke a circuit to produce the electrical pulses used for coded messages. By the late 1800s, telegraph systems also used knife-style line switches and other contact mechanisms to connect, disconnect, or reroute circuits.

As electrical systems grew, switches became specialized for different jobs: knife and plug switches for routing circuits, pushbuttons and toggles for human control, rotary selectors for choosing among many connections, and automatic switches for machinery and safety systems.

Relays extended the same basic idea by allowing an electrical signal to operate a separate set of contacts. That made it possible to repeat signals and, much later, to build control and calculating circuits from large numbers of electrically operated switches. In 1937 Claude Shannon's master's thesis showed how Boolean algebra could be applied systematically to relay and switching circuits, an important bridge between telephone switching and modern digital logic.

Historical reference points: the Smithsonian's National Museum of American History identifies telegraph keys as electrical on-off switches and preserves an 1871 Western Electric double-throw telegraph cut-off switch. The Computer History Museum describes Shannon's 1937 work applying Boolean algebra to relay switching circuits.

Switch pages

• Mechanical switch types
• Poles, throws, contact states, symbols, and ratings
• Limit, microswitch, reed, and sensing switches
• Electronic and digital switching
• Relays