Switches

Also see: Relays | Series & Parallel

Selecting a Switch

Features to consider when selecting a switch:

• Type of contacts such as DPDT.
• Ratings for voltage and current.
• Method of operation toggle, slide etc.

The following terms are used to identify different types of standard switches:

SPST = Single Pole, Single Throw
SPDT = Single Pole, Double Throw
DPST = Double Pole, Single Throw
DPDT = Double Pole, Double Throw

Switch Contacts

Several terms are used to describe switch contacts:

• Pole - number of switch contact sets.
• Throw - number of conducting positions (only used for single and double)
• Way - number of conducting positions.
• Momentary - switch returns to its normal position when released.
• Open - off position, contacts not conducting.
• Closed - on position, contacts conducting, there may be several on positions.

A simple on-off switch

A simple on-off switch has one set of contacts, single pole, and one switching position which conducts, single throw. This is type of switch is called SPST (single pole, single throw) and its action is described as ON-OFF. The switch mechanism has two positions: closed (on) and open (off), but it is called 'single throw' because only one position conducts.

A simple push switch

A simple push-switch, such as one for a doorbell, has one set of contacts and the on position is only momentary, as soon as you release the switch it goes back to off. This action is called push-to-make (push to close contacts). The momentary action is shown by using brackets like this: (ON)-OFF.

Switch Contact Ratings

Switch contacts are rated with a maximum voltage and current, and there may be different ratings for AC and DC. The AC values are higher because the current falls to zero many times each second and an arc is less likely to form across the switch contacts.

For low voltage electronics projects the voltage rating will not matter, but you may need to check the current rating. The maximum current is less for inductive loads (coils and motors) because they cause more sparking at the contacts when switched off.

Standard Switches

ON-OFF, SPST

SPST = Single Pole, Single Throw

A simple on-off switch.

This type can be used to switch the power supply to a circuit. The photograph shows a SPST toggle switch

When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral.

(ON)-OFF, Push-to-make, SPST Momentary

A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around (ON). This is the standard doorbell switch.

ON-(OFF), Push-to-break, SPST Momentary

A push-to-break switch returns to its normally closed (on) position when you release the button, this is shown by the brackets around (OFF).

ON-ON, SPDT

SPDT = Single Pole, Double Throw

This switch can be on in both positions, switching on a separate device in each case. It is also called a changeover switch.

For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position.

A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled.

Toggle, Slide and Rocker SPDT switches

ON-OFF-ON, SPDT Centre Off

This is a special version of the standard SPDT switch shown above. It has a third switching position in the centre which is off.

Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. The brackets are used to show the momentary action.

Dual ON-OFF, DPST

DPST = Double Pole, Single Throw

A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol).

A DPST switch is often used for mains electricity because it switches both the live and neutral connections.

Dual ON-ON, DPDT

DPDT = Double Pole, Double Throw

A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol).

Reversing Switch

A DPDT switch can be wired up as a reversing switch for a motor as shown in the diagram below:

ON-OFF-ON, DPDT Centre Off

This is a special version of the standard DPDT switch shown above. It has a third switching position in the centre which is off. This can be useful for motor control because you have forward, off and reverse positions.

Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. The brackets are used to show the momentary action.

Special Switches

Push-Push Switch (e.g. ON-OFF, SPST)

This looks like a momentary action push switch but it is a standard SPST on-off switch: push once to switch on, push again to switch off. This is called a latching action.

Microswitch (usually ON-ON, SPDT)

Microswitches are designed to switch fully open or fully closed in response to small movements and small forces. They are available with levers and rollers attached.

Microswitches are often used as sensors in machinery to detect the position of parts including doors, for example they may be used to stop a machine if a door or panel is opened which exposes moving parts.

Normal switches are likely to suffer from damaging arcing (sparking) at their contacts when they are not fully open or closed, microswitches are designed to avoid this problem.

Keyswitch

A key operated switch. The example shown is SPST.

Tilt Switch (SPST)

Tilt switches contain a conductive liquid and when tilted this bridges the contacts inside, closing the switch. They can be used as a sensor to detect the position of an object. Some tilt switches contain mercury which is poisonous.

Reed Switch

The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available.

Warning: reed switches have a glass body which is easily broken! For advice on handling please see the Electronics in Meccano website.

DIP Switch

DIP = Dual In-line Parallel.

A DIP switch is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit.

DIP switches are used to set up circuits, for example setting the code of a remote control.

Multi-pole Switch

The picture shows a 6-pole double throw switch, also known as a 6-pole changeover switch. It can be set to have momentary or latching action. Latching action means it behaves as a push-push switch, push once for the first position, push again for the second position etc.

Multi-way Switch

Multi-way switches have 3 or more conducting positions and they may have several poles (contact sets).

The symbol shows a 1-pole 4-way switch.

A popular type has a rotary action and it is available with a range of contact arrangements from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions) may be reduced by adjusting a stop under the fixing nut. For example if you need a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop.

Contrast a multi-way switch (many switch positions) with a multi-pole switch (many contact sets) described above.

Rapid Electronics have kindly allowed me to use their images on this website and I am very grateful for their support. They stock a wide range of switches and other components for electronics and I am happy to recommend them as a supplier.