Switch (SPST) Symbol
Definition: The Switch (SPST) symbol represents a Single-Pole Single-Throw switch — a two-terminal device that either completes or breaks a single current path — drawn as a short angled line (the actuator blade) bridging or lifted away from a contact point between two lead lines, per IEC 60617-07 and ANSI Y32.2 / IEEE 315-1975, with the open state indicating no current flow and the closed state indicating a direct conductive connection; it is assigned the schematic designator S or SW.
Also known as: SPST switch symbol, single-pole single-throw symbol, toggle switch symbol, on-off switch symbol, S symbol schematic, SW symbol circuit diagram, make contact symbol.
What the Switch (SPST) symbol means
The SPST switch symbol denotes the simplest possible switching element: one movable contact that can connect or disconnect a single pair of terminals. When the blade (actuator) in the symbol is shown touching or bridging the contact, the switch is closed (ON) — current can flow between pin 'In' and pin 'Out'. When the blade is shown lifted away at an angle, the switch is open (OFF) — no current can flow and the circuit is broken.
The SPST designation stands for Single-Pole (one contact set) Single-Throw (one possible closed position). This contrasts with SPDT (single-pole double-throw, which can connect to one of two alternative circuits) and DPDT (double-pole double-throw). The SPST switch symbol is the foundation from which all other switch symbols are derived, and it appears in virtually every circuit diagram that includes user control, power switching, or logic input.
How to identify the Switch (SPST) symbol
The SPST switch symbol is drawn as two short horizontal lead lines (representing the In and Out terminals) with a short angled or diagonal line — the switch blade or actuator — shown either resting on the right-hand contact (closed) or angled upward from it at roughly 30–60° (open). In the most common open-state representation, the left lead ends in a filled circle (the pivot point), the blade is a diagonal line rising from the pivot, and the right lead ends in a small gap below the blade tip. In IEC 60617-07, the basic make contact (normally open) symbol is a horizontal line gap with a short diagonal bridge line shown open. In ANSI Y32.2 / IEEE 315, the symbol is similar — a pivot point, angled blade, and contact gap. A normally closed (NC) SPST variant shows the blade already touching the contact, with a diagonal slash across it or without separation, to indicate the default closed state.
Function in a circuit
An SPST switch controls whether current flows between its two terminals (In and Out). In the closed (ON) state, the switch presents a near-zero resistance path, allowing full circuit current to flow. In the open (OFF) state, the switch presents a theoretically infinite resistance, breaking the circuit and stopping all current flow. The switch itself does not consume or convert energy in either state — it simply controls the conduction path. SPST switches are used as power on/off controls, manual inputs to logic circuits, safety interlocks, limit switches in mechanical systems, and reed switches actuated by magnetic fields.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines switch and contactor symbols. The basic make contact (normally open) symbol — which is the SPST open default — is IEC 60617-07-01-01: a gap in a conductor with a short diagonal line above representing the open blade. The basic break contact (normally closed) is IEC 60617-07-01-02. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the SPST switch with a pivot-point filled circle on the left lead, a diagonal blade line, and the right contact line. The normally open (NO) form shows the blade lifted; the normally closed (NC) form shows the blade in contact. The designator S (or SW) is assigned by IEEE 315. |
| Key difference | IEC 60617-07 and ANSI Y32.2 / IEEE 315-1975 both show the SPST switch as a pivoted blade next to a contact gap, and the visual forms are very similar. The IEC symbol tends to be more schematic/abstract (a simple gap and line), while ANSI symbols often include a filled circle at the pivot. In practice the two forms are used interchangeably in EDA tools and are rarely confused. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
The SPST switch symbol carries no inherent electrical magnitude. Switch specifications noted adjacent to the symbol include: voltage rating (maximum voltage the open switch can withstand, e.g. 250 VAC or 30 VDC), current rating (maximum current through the closed contacts, e.g. 5 A, 10 A), contact resistance when closed (typically < 50 mΩ for quality switches), insulation resistance when open (typically > 100 MΩ), and actuation force or operating temperature range for mechanical types. Relay-driven switch contacts may also show coil voltage.
Where the Switch (SPST) symbol is used
- Power on/off switch in any battery-powered or mains-powered device — the first element in series with the supply after the battery or fuse
- Push-button inputs on microcontroller circuits, where one terminal connects to a GPIO pin via a pull-up resistor and the other connects to GND
- Safety interlock switches in industrial machinery that cut power when a guard door is opened, preventing access to moving parts while energised
- Reed switches in proximity sensors and door/window alarm contacts, actuated magnetically without physical contact
- Limit switches on CNC machines and 3D printers that signal the end of axis travel to the motion controller
- Test points and jumper links on PCBs, representing a solder-bridge or header jumper that can open or close a circuit path
Example
In a battery-powered LED night-light schematic, a 5 A / 30 VDC SPST toggle switch (S1) is placed in series between the positive terminal of a 3 V coin-cell battery (BT1) and a 100 Ω current-limiting resistor (R1). The switch symbol (open blade, In terminal to battery positive, Out terminal to resistor) shows that closing S1 completes the circuit, lighting the white LED (D1). The '5A / 30V' annotation beside S1 confirms the switch is rated well above the actual operating current of ~10 mA, providing a large safety margin.
Key facts
- The SPST (Single-Pole Single-Throw) switch symbol is a pivoted angled blade next to a contact gap between two leads, representing a component that either completes or breaks a single current path.
- The switch symbol has two terminals: In (pin id 'in') on the left lead and Out (pin id 'out') on the right lead; when closed, current flows freely between them; when open, no current flows.
- The schematic designator for a switch is S or SW, as defined in IEEE 315-1975; individual switches are labelled S1, S2, S3 in sequence.
- SPST means Single-Pole Single-Throw: one pole (one contact set) with one throw (one possible closed position). It is the simplest switch type and the basis for all more complex configurations (SPDT, DPST, DPDT).
- The normally open (NO) SPST symbol shows the blade lifted away from the contact — the switch is open by default and closes when actuated. The normally closed (NC) variant shows the blade in contact — closed by default, opens when actuated.
- The SPST switch symbol is defined in IEC 60617-07 (make/break contact symbols) and ANSI Y32.2-1975 / IEEE 315-1975; the visual forms are nearly identical between the two standards.
- A physical SPST switch in the open state presents near-infinite impedance (circuit broken); in the closed state it presents near-zero resistance (typically < 50 mΩ for quality contacts).
- The SPST switch symbol is non-polar — it can be connected in either orientation without affecting function, because it simply opens or closes the conduction path between its two terminals.
Diagrams that use this symbol
- circuit diagram
- and circuit diagram
- wiring diagram
- writing diagram
- wiring diagram for light switch
- light switch connection diagram
- relay diagram
- relay wiring diagram
Frequently asked questions
What does the SPST switch symbol look like?
The SPST switch symbol looks like a short diagonal line (the switch blade) pivoting from the left terminal, with the right terminal shown as a separate line to which the blade either connects (closed) or does not connect (open). The open-state symbol is the most common: the blade is angled upward, leaving a visible gap between it and the right contact, indicating the circuit is broken.
What does the switch SPST symbol mean in a circuit diagram?
The SPST switch symbol means there is a single on/off control point in the circuit. When the symbol shows the blade touching the contact (closed), current flows between the In and Out terminals. When the blade is lifted (open), the circuit is broken and no current flows. SPST stands for Single-Pole Single-Throw — one contact set, one position.
What is the difference between a normally open and normally closed SPST switch symbol?
A normally open (NO) SPST switch symbol shows the blade lifted away from the contact — the switch is open (off) in its default, un-actuated state. A normally closed (NC) SPST switch symbol shows the blade already touching the contact — the switch is closed (on) by default and opens when actuated. Most basic on/off switches use the normally open form.
What is the designator letter for a switch on a schematic?
The designator letter for a switch is S (or SW in some conventions), as defined in IEEE 315-1975. Individual switches are labelled S1, S2, S3 in sequence. The designator appears next to the switch symbol on the schematic and in the bill of materials.
What is the difference between the IEC and ANSI SPST switch symbols?
IEC 60617-07 and ANSI Y32.2 / IEEE 315-1975 both represent the SPST switch as a pivoted blade next to a contact gap, and the two forms are very similar. IEC uses a more schematic line representation; ANSI frequently adds a filled dot at the pivot point. In practice, CAD tools and most schematics use the two forms interchangeably without confusion.
What does SPST stand for in a circuit symbol?
SPST stands for Single-Pole Single-Throw. Single-Pole means the switch controls one circuit path (one pair of contacts). Single-Throw means there is only one possible closed position — the switch either connects or disconnects those two contacts, with no alternative routing. SPST is the simplest possible switch configuration and is used for straightforward on/off control.
What are the terminals on the SPST switch symbol?
The SPST switch symbol has exactly two terminals: the In terminal (pin id 'in') on the left lead, which typically connects toward the power source, and the Out terminal (pin id 'out') on the right lead, which connects toward the load. The switch is non-polar — it can be connected in either orientation — but by convention the supply-side terminal is labelled In and the load-side terminal is labelled Out.
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