Understanding the SPST Relay: Simple Switching Power
If you’ve ever worked with electronics or automation, you’ve likely heard of a relay. A relay is basically an electrically controlled switch — and one of the simplest and most useful types is the SPST relay.
What Does SPST Mean?
SPST stands for Single Pole Single Throw. It’s the most straightforward relay configuration, featuring one input (pole) and one output (throw). Essentially, it connects or disconnects a single circuit when activated — just like a basic ON/OFF switch.
How an SPST Relay Works
An SPST relay uses an electromagnet to open or close contacts. When the control coil is energized, it pulls a metal armature that closes the circuit. When the coil is de-energized, the circuit opens again. This allows low-power control signals (like from an Arduino or sensor) to control higher-power loads (like motors or lights).
In simple terms: apply voltage to the coil → the relay clicks → current flows to the load. Remove voltage → the connection opens.
Common Uses
- Turning lights or motors on and off
- Isolating high-voltage loads from low-voltage control circuits
- Automotive or industrial control panels
- Home automation and IoT projects
SPST Relay Schematic Symbol
The schematic symbol for an SPST relay looks like a simple switch symbol (open or closed) combined with a coil symbol, showing how the electromagnet controls the contacts.
Wiring Example
In an Arduino or microcontroller setup, the SPST relay can be connected through a relay module:
VCC → 5V
GND → GND
IN → Digital Pin (control signal)
COM → Power supply input
NO → Load output (e.g., lamp or motor)
When the digital pin goes HIGH, the relay coil energizes, connecting COM and NO to power your load.
Advantages of SPST Relays
- Simple operation and wiring
- Reliable control of higher currents
- Excellent for beginners learning automation circuits
- Can safely isolate control and load sides
Conclusion
The SPST relay is a fundamental switching device in electronics. Whether you’re automating lights, controlling motors, or experimenting with sensors, understanding how an SPST relay works gives you the foundation to build more advanced relay and control systems.