Switches
A switch is an electrical or electronic component used to open, close or change an electrical circuit. Switches are found in almost every type of electronic and electrical equipment, from simple push buttons and power switches to complex rotary selectors and multi-position control switches.
What Is a Switch?
A switch controls the flow of electrical current by changing the electrical connection between one or more terminals.
The simplest switch has two states:
- Open — the circuit is interrupted.
- Closed — the circuit is electrically connected.
OPEN
───── ─────
\ /
\ /
gap
CLOSED
───────────────
How a Switch Works
A mechanical switch normally contains conductive contacts that can be moved relative to each other.
When the contacts touch, current can flow. When they separate, the circuit is interrupted.
Switch
│
▼
Source ──/ ── Load
│
▼
Control
Normally Open Switch
A normally open switch, abbreviated NO, is open when it is in its normal, unactivated state.
Activating the switch closes the electrical connection.
Normal:
────── / ──────
OPEN
Pressed:
───────────────
CLOSED
Push buttons used to activate functions are commonly normally open.
Normally Closed Switch
A normally closed switch, abbreviated NC, is closed when it is in its normal state.
Activating the switch opens the electrical connection.
Normal:
───────────────
CLOSED
Activated:
────── / ──────
OPEN
Normally closed switches are commonly used in safety and limit-switch applications.
NO vs NC
| Type | Normal State | When Activated |
|---|---|---|
| Normally Open | Open | Closes |
| Normally Closed | Closed | Opens |
Single-Pole Switch
A single-pole switch controls one electrical circuit.
A common example is a simple two-terminal ON/OFF switch.
Terminal 1 ──/ ── Terminal 2
SPST Switch
SPST means Single Pole Single Throw.
It is one of the simplest switch configurations and provides a single ON/OFF function.
┌──── Switch ────┐
│ │
──────o/ o─────────────
SPST switches are commonly used as power switches and simple control switches.
SPDT Switch
SPDT means Single Pole Double Throw.
It has one common terminal that can be connected to either of two outputs.
┌── Output A
Common ──────o
└── Output B
SPDT switches are useful for selecting between two circuits or signals.
DPST Switch
DPST means Double Pole Single Throw.
It allows two independent circuits to be switched simultaneously.
Circuit A ──/ ──
Circuit B ──/ ──
│
One control
DPDT Switch
DPDT means Double Pole Double Throw.
It contains two independently switched poles, each capable of selecting between two contacts.
DPDT switches can be used for polarity reversal and other changeover applications.
Push Button Switch
A push button switch changes state when its button is pressed.
Push buttons are commonly used for:
- Reset
- Start
- Stop
- Menu controls
- Doorbells
- Control panels
Momentary Switch
A momentary switch remains in its activated state only while the actuator is being pressed or held.
Once released, the switch returns to its normal state.
Maintained Switch
A maintained switch remains in its new state after the actuator is released.
A typical ON/OFF toggle switch is a maintained switch.
Toggle Switch
A toggle switch uses a mechanical lever to change its electrical state.
Toggle switches are available in configurations such as:
- SPST
- SPDT
- DPST
- DPDT
- Multiple-position configurations
Rocker Switch
A rocker switch uses a rocking actuator instead of a lever.
Rocker switches are widely used as equipment power switches.
Many illuminated power switches use the rocker-switch format.
Slide Switch
A slide switch changes its state by moving a small actuator along a track.
Slide switches are commonly used for:
- Power selection
- Mode selection
- Configuration
- Small electronic equipment
Rotary Switch
A rotary switch selects one of several positions by rotating an actuator.
Position 1
│
▼
┌─────────┐
Position 2 ─│ ROTARY │─ Position 3
│ SWITCH │
└─────────┘
│
▼
Position 4
Rotary switches are useful when several circuits or settings need to be selected.
Rotary Selector Switch
Rotary selector switches are commonly used to select:
- Input sources
- Operating modes
- Voltage ranges
- Resistance ranges
- Fan speeds
- Audio channels
- Measurement ranges
Tactile Switch
A tactile switch is a small momentary push button designed to provide a distinct mechanical click when pressed.
Tactile switches are widely used on:
- Keypads
- Remote controls
- Microcontroller boards
- Consumer electronics
- Control panels
Microswitch
A microswitch is a snap-action switch designed to change state quickly when its actuator reaches a particular position.
Microswitches are commonly used in:
- Limit switches
- Printers
- Appliances
- Machines
- Door detection
- Mechanical position sensing
Limit Switch
A limit switch detects mechanical movement or position.
A machine can use a limit switch to determine whether a moving part has reached a specific position.
Moving Mechanism
│
▼
Actuator
│
▼
Limit Switch
│
▼
Controller
Reed Switch
A reed switch is an electrically operated switch containing contacts inside a sealed glass or similar protective enclosure.
The contacts respond to a magnetic field.
Reed switches are used for:
- Door sensors
- Position sensing
- Security systems
- Speed sensing
- Magnetic detection
Magnetic Switch
A magnetic switch changes state in response to a magnetic field.
A reed switch is one common implementation of a magnetic switch.
DIP Switch
A DIP switch is a group of small switches mounted in a compact package.
DIP switches are commonly used for hardware configuration.
┌────────────────────┐ │ 1 2 3 4 5 6 │ │ ● ● ● ● ● ● │ └────────────────────┘
They can be used to configure addresses, operating modes and other settings.
Key Switch
A key switch requires a physical key to operate the switch.
It can be used to restrict access to equipment or control systems.
Selector Switch
A selector switch allows an operator to choose between different operating modes.
Examples include:
- OFF / LOW / HIGH
- Manual / Automatic
- Input 1 / Input 2 / Input 3
- Mode 1 / Mode 2 / Mode 3
Illuminated Switch
An illuminated switch contains a light source, commonly an LED or neon-type indicator depending on the design.
The illumination can indicate whether a circuit or function is active.
The lamp or LED may have separate electrical terminals from the switch contacts.
Foot Switch
A foot switch allows a circuit to be controlled using a foot rather than a hand.
Foot switches are commonly used with:
- Musical instruments
- Industrial machines
- Power tools
- Medical equipment
- Control systems
Push Button Contact Types
Push buttons can have different contact arrangements.
- Normally open
- Normally closed
- Changeover
- Momentary
- Maintained
Switch Contact Bounce
Mechanical switches do not always make a perfectly clean electrical transition.
When contacts close or open, they can briefly make and break the connection several times.
Ideal:
───────┐
└────────
Real switch:
───────┐_┌_┌────
└─┘ └────
This behavior is called contact bounce.
Switch Debouncing
Electronic circuits and microcontrollers can use debouncing techniques to prevent contact bounce from being interpreted as multiple button presses.
Common methods include:
- Software debounce
- RC filtering
- Schmitt-trigger circuits
- Dedicated debounce circuits
Switch Contact Resistance
A closed switch has a small contact resistance.
For low-current signal applications, even small changes in contact resistance can sometimes become important.
Dirty or worn contacts can produce higher resistance.
Switch Current Rating
Every switch has a specified current rating.
The rating depends on factors such as:
- Contact construction
- Contact material
- Voltage
- Load type
- Temperature
- Switching frequency
A switch should not be selected based only on physical size.
Switch Voltage Rating
Switches also have maximum voltage ratings.
The voltage rating depends on the contact spacing, insulation, materials and switching conditions.
A switch rated for low-voltage DC should not automatically be assumed suitable for mains voltage.
AC vs DC Switch Ratings
A switch may have different ratings for AC and DC loads.
DC can be particularly demanding for mechanical contacts because the current does not naturally pass through zero as it does during each cycle of AC.
Always use the manufacturer's specified rating for the actual load.
Resistive Loads
Resistive loads, such as certain heaters, generally produce relatively predictable switching currents.
The switch must still be rated for the actual voltage and current.
Inductive Loads
Inductive loads include motors, relays, transformers and coils.
When an inductive load is switched off, the stored magnetic energy can produce a voltage spike.
Switch OFF
│
▼
Inductive Load
│
▼
Voltage Spike
│
▼
Switch Contacts This can cause arcing and shorten contact life.
Switch Arcing
When switch contacts separate while current is flowing, an electrical arc can form between the contacts.
Arcing can:
- Damage contacts
- Produce heat
- Cause electrical noise
- Reduce switch life
The effect is particularly important with inductive and high-current loads.
Switching an Inductive Load
Suitable suppression components may be required when switching inductive loads.
Depending on the circuit, protection can include:
- Flyback diode for suitable DC coils
- RC snubber
- TVS protection
- Other load-specific suppression circuits
The appropriate protection depends on the load and switching circuit.
Power Switch
A power switch is designed to control the supply of electrical power to equipment.
Power switches must be selected according to the actual voltage, current and load characteristics.
Signal Switch
A signal switch controls a low-power electrical signal rather than directly switching a large power load.
Signal switches are commonly found in:
- Audio equipment
- Measurement equipment
- Control circuits
- Communication equipment
Audio Switches
Switches are widely used in audio equipment to select signal paths, inputs and operating modes.
Examples include:
- Input selection
- Speaker selection
- Mute control
- Source selection
- Mode selection
Electronic Switches
Not all switches are mechanical.
Electronic switching can be performed using semiconductor devices such as:
- MOSFETs
- BJTs
- IGBTs
- Analog switches
- Solid-state relays
These devices have no mechanically moving contacts.
Mechanical vs Electronic Switches
| Feature | Mechanical | Electronic |
|---|---|---|
| Moving contacts | Usually yes | No |
| Contact bounce | Possible | No mechanical bounce |
| Physical wear | Possible | Different failure mechanisms |
| Switching speed | Limited | Can be very high |
| Galvanic isolation | Possible | Depends on circuit |
Switch Wiring
A simple SPST switch can be connected in series with a load.
+ Supply │ │ ▼ Switch │ ▼ Load │ ▼ Ground
Opening the switch interrupts the current path.
Switch on the Positive Supply
In many low-voltage circuits, a switch can be placed in series with the positive supply.
+V ─── Switch ─── Circuit │ GND
The correct switching arrangement depends on the circuit design.
Switch on the Ground Side
A switch can also be placed in the return path in some circuits.
+V ───── Circuit │ ▼ Switch │ GND
However, switching the ground side can have undesirable consequences in some systems, so the circuit design should be considered carefully.
Switch Input to a Microcontroller
A mechanical push button can be connected to a microcontroller input.
+V │ Resistor │ ├────────► MCU Input │ Switch │ GND
The input resistor provides a defined logic state when the switch is not activated.
Pull-Up and Pull-Down Resistors
Microcontroller inputs should generally not be left floating when a switch is used to select a logic state.
A pull-up or pull-down resistor provides a defined default state.
Pull-up: +V │ R │ ├──── MCU Input │ Switch │ GND
When the switch is open, the input is pulled high. When the switch closes, the input is pulled low.
Switch Testing
A multimeter can be used to test many mechanical switches.
- Disconnect the equipment from power.
- Identify the switch terminals.
- Set the multimeter to continuity or resistance mode.
- Measure the switch in its normal state.
- Activate the switch.
- Measure again.
Testing a Normally Open Switch
A normally open switch should normally show an open circuit.
When activated, it should show continuity or a low resistance.
Not pressed → OPEN Pressed → CLOSED
Testing a Normally Closed Switch
A normally closed switch should normally show continuity.
When activated, the circuit should open.
Not pressed → CLOSED Pressed → OPEN
Testing a Multi-Position Switch
For rotary and multi-position switches, determine which terminals are connected at each position.
A continuity meter can be used to map the switch contacts.
Position 1 → Common + Contact 1 Position 2 → Common + Contact 2 Position 3 → Common + Contact 3
Switch Troubleshooting
When a switch-controlled circuit does not operate, check:
- Switch mechanical operation.
- Switch terminals.
- Continuity.
- Solder joints.
- Supply voltage.
- Load connection.
- Control wiring.
Switch Has No Continuity
If a switch should be closed but shows an open circuit, possible causes include:
- Damaged contacts
- Broken internal mechanism
- Corrosion
- Broken terminal
- Incorrect switch identification
Switch Has Permanent Continuity
If a switch remains closed when it should open, possible causes include:
- Welded contacts
- Contamination
- Mechanical failure
- Incorrect terminal selection
- Internal short
Switch Feels Loose
A loose actuator can indicate mechanical wear or damage.
If the switch is mounted to a PCB or panel, inspect the mounting hardware and solder joints as well.
Switch Replacement
When replacing a switch, match both its electrical and mechanical characteristics.
Check:
- Switch type
- Number of poles
- Number of throws
- Normally open or normally closed contacts
- Momentary or maintained operation
- Current rating
- Voltage rating
- AC/DC rating
- Panel dimensions
- PCB footprint
- Terminal arrangement
How to Identify a Switch
- Count the terminals.
- Determine which terminals are connected when inactive.
- Activate the switch.
- Determine which terminals become connected.
- Identify the actuator type.
- Check physical dimensions.
- Check mounting method.
- Look for manufacturer markings.
How to Select a Switch
- Determine the circuit voltage.
- Determine the load current.
- Determine whether the load is AC or DC.
- Determine whether the load is resistive or inductive.
- Choose the required pole and throw configuration.
- Choose momentary or maintained operation.
- Determine the required actuator style.
- Check the physical mounting dimensions.
- Check environmental requirements.
- Verify the manufacturer's electrical ratings.
Switch Contact Life
Mechanical switches have a finite contact life.
The expected life depends on:
- Load current
- Load voltage
- Load type
- Switching frequency
- Contact material
- Environmental conditions
Switching an inductive or high-current load can reduce contact life compared with a low-current resistive load.
Switch Environmental Ratings
Switches may have ratings for environmental conditions such as:
- Operating temperature
- Humidity
- Dust
- Water exposure
- Vibration
- Mechanical shock
For outdoor or industrial equipment, select a switch with an appropriate environmental rating.
IP-Rated Switches
Some switches are designed with ingress protection against dust and water.
The appropriate IP rating depends on the installation environment.
Switches in Audio Equipment
Audio equipment can contain many different switches.
- Power switches
- Input selectors
- Speaker selectors
- Mute switches
- Mode switches
- Push-button controls
- Rotary selectors
Signal switches used in audio circuits should be selected with suitable contact characteristics to avoid unwanted noise and distortion.
Switches in Amplifiers
Amplifiers commonly use switches for power control, input selection, speaker selection and operating modes.
Input 1 ──┐ │ Input 2 ──┼──► Selector Switch ───► Amplifier │ Input 3 ──┘
Switches in Power Supplies
Power supplies can use switches for:
- Power ON/OFF
- Voltage selection
- Current-range selection
- Output enable
- Mode selection
Any switch connected to hazardous voltage must be appropriately rated and installed according to applicable safety requirements.
Switches in Microcontroller Projects
Push buttons and toggle switches are commonly connected to microcontrollers for user input.
The microcontroller detects the switch state through a digital input.
Software can then respond to the detected state.
Switches in Control Panels
Industrial control panels commonly combine switches, selector switches, push buttons and indicator lamps.
Operators can use them to start, stop and configure machinery.
Switch Faults
| Symptom | Possible Cause |
|---|---|
| No continuity | Broken contacts, corrosion or mechanical failure |
| Intermittent operation | Dirty contacts, worn contacts or loose connection |
| High resistance when closed | Contamination, oxidation or damaged contacts |
| Switch becomes hot | Excessive current or insufficient contact rating |
| Contacts arc | High current or inductive load |
| Actuator does not move correctly | Mechanical damage or wear |
| Wrong circuit selected | Incorrect switch wiring or internal contact fault |
Advantages of Mechanical Switches
- Simple operation
- Easy to understand
- Low cost
- Can provide physical feedback
- Can switch many different types of circuits
- Available in many configurations
- Can provide galvanic isolation between contacts
Limitations of Mechanical Switches
- Mechanical wear
- Contact bounce
- Contact resistance
- Arcing with some loads
- Limited switching speed
- Physical size can be significant
- Contacts can become contaminated
Common Switch Design Mistakes
- Using a switch below the required voltage rating
- Using a switch below the required current rating
- Ignoring the difference between AC and DC ratings
- Ignoring inductive-load switching requirements
- Using the wrong NO/NC contacts
- Failing to debounce a mechanical switch in digital circuits
- Ignoring mechanical dimensions during replacement
- Assuming all switches with the same appearance have the same pinout
- Failing to provide adequate protection for high-current loads
Key Points
- A switch controls an electrical circuit by opening, closing or changing connections.
- NO switches are normally open and close when activated.
- NC switches are normally closed and open when activated.
- SPST provides a simple ON/OFF function.
- SPDT selects between two circuits or outputs.
- Push buttons can be momentary or maintained.
- Toggle, rocker, slide and rotary switches are common mechanical switch types.
- Microswitches provide snap-action switching and are often used for position detection.
- Reed switches respond to magnetic fields.
- Mechanical switches can suffer from contact bounce, wear, corrosion and arcing.
- Switch ratings must match the actual voltage, current and load type.
- Inductive loads can require suppression to protect switch contacts.
- A multimeter can be used to test switch continuity and contact operation.
- Always verify the switch configuration and ratings before replacement.