Relay Module Symbol
Definition: The Relay Module symbol represents a single-channel electromechanical relay on a breakout PCB, depicted in circuit diagrams with a Coil + and Coil − control input and three SPDT load-switching pins (NO, COM, NC), used to switch mains-voltage or high-current loads from a microcontroller or low-power logic signal.
Also known as: single channel relay module, relay breakout board, 5V relay module, Arduino relay module, relay shield.
What the Relay Module symbol means
The Relay Module symbol denotes a self-contained PCB module housing one electromechanical relay with associated driver circuitry (usually a transistor and flyback diode) and an indicator LED. The coil side (Coil +, Coil −) accepts a low-power drive signal from a microcontroller GPIO or transistor, while the contact side provides isolated SPDT switching for a high-voltage or high-current load circuit.
In circuit diagrams the relay module symbol separates the low-voltage control circuit (left side: Coil +, Coil −) from the high-voltage load circuit (right side: NO, COM, NC). This visual separation reminds the designer and reader that the two circuits are electrically isolated through the electromagnetic coupling of the relay coil and armature.
How to identify the Relay Module symbol
The Relay Module symbol is drawn as a rectangle with two control pins (Coil +, Coil −) entering from the left, and three load-switching pins (NO, COM, NC) exiting from the right. Inside the rectangle, a small coil symbol and a switching arm (contact) are visible, matching the IEC 60617 relay representation. The enclosing rectangle indicates a module (integrated PCB) rather than a discrete relay component.
Function in a circuit
A relay module switches a load circuit on or off in response to a coil energisation signal. When voltage is applied between Coil + and Coil −, the electromagnet attracts the armature, moving the contact from the Normally Closed (NC) pin to the Normally Open (NO) pin, both referenced to the COM (common) pin. The load circuit connected between COM and NO closes, while the load circuit between COM and NC opens. When the coil is de-energised, spring tension returns the armature to the NC position. The flyback diode on the module absorbs the coil's back-EMF spike, protecting the driving transistor or GPIO.
Standards: IEC vs ANSI
| IEC 60617 | Individual relay components are specified in IEC 61810 (elementary relays). The relay coil and contact symbols within the module follow IEC 60617-07 (switching devices). IEC 60947-5-1 covers auxiliary contacts applicable to relay module contact ratings. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 defines relay coil symbols (rectangle with coil annotation) and contact symbols (NO, NC per IEEE 315 Section 13). The module's enclosing rectangle follows IEEE 315 block symbol conventions. |
| Key difference | IEC 60617 draws the relay coil as a rectangle with terminal lines; IEEE 315 also uses a rectangle but labels it with device type (e.g. 'K' for relay). Both standards define NO contacts as an open-gap line pair and NC contacts with a diagonal slash. The visual representation is nearly identical between IEC and ANSI for relay modules. |
Terminals / pins
| Pin | Name |
|---|---|
| coil_pos | Coil + |
| coil_neg | Coil - |
| no | NO |
| com | COM |
| nc | NC |
Typical values
Coil voltage: 5 V DC (most common), 12 V DC, 24 V DC. Coil current: 70–90 mA (5 V version). Contact rating: 10 A at 250 V AC, 10 A at 30 V DC. Contact configuration: SPDT (Form C) — COM, NO, NC. Isolation: electrical isolation via electromagnetic coupling (galvanic); some modules add optocoupler isolation. Designator: K (relay).
Where the Relay Module symbol is used
- Arduino and Raspberry Pi projects switching 120/240 V AC household appliances from 5 V GPIO outputs
- IoT smart home switches controlling lighting, fans, and plug sockets remotely
- Automotive and 12 V DC systems switching starter solenoids, horns, or additional headlights
- Industrial prototype control panels for relay-logic interlocking and sequencing
- Battery management systems switching charge/discharge paths under microcontroller control
- Alarm and security systems controlling sirens, door strikes, and gate operators
Example
In an Arduino home automation schematic, the Relay Module symbol's Coil + pin connects through a 1N4007 flyback diode to Arduino GPIO D3, and Coil − connects to Arduino GND. The NO pin connects to the live conductor of a 230 V AC lamp circuit; COM connects to the mains supply live bus; when D3 goes HIGH the lamp switches on.
Key facts
- The Relay Module symbol has five pins: Coil + and Coil − (low-voltage control side) and NO, COM, and NC (high-voltage SPDT load-switching side), with electromagnetic isolation between the two sides.
- When the coil is de-energised (power off), COM is connected to NC (Normally Closed) — the load on the NC path is active without coil power.
- When the coil is energised, COM transfers to NO (Normally Open) — the load on the NO path activates and the NC path opens.
- Most relay modules include a built-in flyback (freewheeling) diode across the coil and a transistor driver, so the GPIO pin only needs to source or sink a few milliamps rather than the full 70–90 mA coil current.
- At 5 V, the relay coil draws approximately 70–90 mA, which may exceed the source capability of some microcontroller GPIO pins (typically 8–40 mA); modules with onboard transistors solve this by using the GPIO to switch the transistor base.
- The relay module provides galvanic isolation between the control circuit and the load circuit via the electromagnetic coupling — this protects the microcontroller from mains voltages if load-side insulation fails.
- The designator letter for a relay on a schematic is K per IEEE 315 and common practice (e.g. K1 for the first relay).
Frequently asked questions
What does the relay module symbol mean in a circuit diagram?
The relay module symbol represents a single-channel electromagnetic switching module with a low-voltage coil side (Coil +, Coil −) and a high-voltage SPDT contact side (NO, COM, NC). It indicates that a microcontroller signal controls the switching of a separate, electrically isolated load circuit.
What do the NO, COM, and NC pins on a relay module do?
COM is the common contact that connects to either NO or NC. When the relay coil is de-energised, COM is connected to NC (Normally Closed). When the coil is energised, COM transfers to NO (Normally Open). The load to be switched connects between COM and the desired state pin.
How do I connect a relay module to an Arduino?
Connect Coil + to an Arduino GPIO pin, Coil − to Arduino GND, and the module's VCC (if present) to Arduino 5 V. Connect the load's live conductor to COM and the destination equipment to NO (if you want the load active when the relay energises) or NC (if you want it active when the relay is off).
What voltage relay module should I use with an Arduino?
Use a 5 V relay module for most Arduino projects — the coil operates from the 5 V rail and the control signal from a 5 V GPIO. For ESP8266 or ESP32 running at 3.3 V, use a 3.3 V relay module or a module with an optocoupler input that accepts 3.3 V logic to avoid GPIO damage.
Does a relay module need a flyback diode?
Most relay modules include an onboard flyback diode across the coil and a transistor driver, so no external diode is needed. If you are driving a bare relay coil directly from a GPIO, a 1N4007 flyback diode wired in reverse-parallel across the coil is essential to suppress the back-EMF spike that occurs when the coil is de-energised.
What is the contact current rating of a typical 5 V relay module?
A standard 5 V relay module uses a relay rated for 10 A at 250 V AC and 10 A at 30 V DC. This supports resistive loads up to 2500 W at 250 V AC. For inductive loads (motors, solenoids), derate to approximately 80% and add a snubber capacitor across the contacts to suppress arcing.
What is the schematic designator for a relay?
The standard designator letter for a relay in a schematic is K (per IEEE 315 and common industry practice). The first relay is labelled K1, the second K2, and so on. In older European schematics, the letter R was sometimes used, but K is now the universally preferred designator.
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