Modding 101: Switches

Wiring Symbol

Photo

ON-OFF
Single Pole, Single Throw = SPST

This is a simple on-off switch that most people use. It is used to switch the power supply to a circuit. You should always put the switch in the live wire.

ON-OFF
Double-pole, Single-throw Switch (DPST)

A switch that connects or disconnects two pairs of terminals at the same time. Let�s say you want to turn off two lights with the same switch. This is what you would use. Each set of terminals would control one light.

Momentary Switches
A push-button switch is considered to be "momentary" if it doesn't stay in place when you remove your finger. They can be either �normally open� (N.O.) or it can be �normally closed� (N.C.)

Example:
Your doorbell button is a "normally open" (N.O.) momentary switch. When you push it, the circuit is connected and so the bell rings. When you release it, it disconnects the circuit, and the bell stops ringing.

(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.

N.O.

ON- (OFF)
Push-to-break = SPST Momentary
A push-to-break switch returns to it�s normally closed (on) position when you release the button.

N.C.

ON-ON
Single Pole, Double Throw = SPDT

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

ON-OFF-ON
SPDT Center Off

It has a third switching position in the center, which is off. Momentary (ON)-OFF- (ON) versions are also available where the switch returns to the central off position when released.

Push-Push Switch (SPST = ON-OFF)

This looks like a momentary action push switch but it is a standard on-off switch. You push once to switch it on, push again to switch off. Also known as a maintained switch.

Key switch

A key operated switch works the same as a SPST switch. Instead of some type of lever you use a key to make your selection.

Micro switch

Micro switches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached to the end of the lever. Usually they come with three terminals. Depending on how you want the switch to act you can make it a N.O. or N.C. switch.