8-Channel Relay Board Symbol
Definition: The 8-Channel Relay Board symbol represents a pre-assembled module containing eight independent electromechanical relays driven by optocoupler-isolated control inputs, depicted in circuit diagrams with a VCC power pin and a GND ground pin to show the 5 V DC logic supply, used in microcontroller-based automation and IoT projects to switch mains-voltage or high-current loads from low-power digital outputs.
Also known as: 8-channel relay module, 8-relay board, 8ch relay shield, eight-relay module, Arduino relay board.
What the 8-Channel Relay Board symbol means
The 8-Channel Relay Board symbol denotes an integrated module that provides eight independently controllable SPDT relay contacts on a single PCB. Each channel contains an optocoupler that isolates the microcontroller's 3.3 V or 5 V logic signal from the relay coil, and each relay provides COM, NO, and NC contacts rated for mains switching (typically 10 A at 250 V AC).
In circuit diagrams, the module symbol shows the shared VCC and GND supply pins for the logic/coil power, while each of the eight IN (input) control pins connects to a microcontroller GPIO. The output side contacts (COM, NO, NC per channel) connect to the load being switched, enabling a single microcontroller to control eight separate AC or DC loads simultaneously through software.
How to identify the 8-Channel Relay Board symbol
The 8-Channel Relay Board symbol is drawn as a wide rectangular block labelled '8-CH RELAY' with eight small relay sub-blocks arranged along the bottom edge, each representing one relay channel. VCC and GND power pins appear on the left side of the enclosure. The compact side-by-side arrangement of eight relay outlines distinguishes this module symbol from a single relay module symbol.
Function in a circuit
The 8-channel relay board allows a microcontroller to switch eight independent high-voltage or high-current loads using low-power digital output signals. Each channel's optocoupler converts the GPIO logic signal (active LOW or active HIGH depending on module type) into a coil drive current; the energised coil closes the NO contact (or opens the NC contact) to connect or disconnect the load. The optoisolation provides electrical separation between the microcontroller ground and the load ground, protecting the microcontroller from mains transients.
Standards: IEC vs ANSI
| IEC 60617 | No specific IEC 60617 symbol exists for multi-channel relay boards as modules; individual relays within the board follow IEC 60617 relay coil and contact symbols. Relay contact ratings are specified per IEC 61810 (elementary relays) and IEC 60947 (low-voltage switchgear). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 does not define a dedicated module-level relay board symbol; the schematic representation uses a labelled box or individual relay symbols per IEEE 315 conventions. Relay coils and SPDT contacts follow standard IEEE 315 notation. |
| Key difference | At the module level, both IEC and ANSI schematics use a labelled rectangular block. Individual relay channel contact symbols within the block follow IEC 60617 or IEEE 315 conventions depending on the drawing standard in use. |
Terminals / pins
| Pin | Name |
|---|---|
| vcc | VCC |
| gnd | GND |
Typical values
Operating voltage (VCC): 5 V DC (standard); 3.3 V versions available. Coil current per channel: ~70–80 mA at 5 V. Relay contact rating: 10 A at 250 V AC, 10 A at 30 V DC (per channel). Number of channels: 8 (SPDT per channel). Trigger logic: active LOW (most common) or active HIGH depending on jumper. Optocoupler: PC817 or equivalent.
Where the 8-Channel Relay Board symbol is used
- Home automation systems (Arduino, ESP32, Raspberry Pi) switching mains lighting, fans, and appliances from software-controlled GPIO outputs
- Greenhouse automation controlling irrigation pumps, grow lights, heaters, and ventilation fans on timed schedules
- Industrial PLC expansion where additional switching channels are needed beyond native relay outputs
- HVAC control panels switching zone valves, damper actuators, and blower motors from thermostat logic
- Test and measurement rigs where automated switching of power rails, loads, or signal paths is required
- Access control systems switching door strikes, gate motors, and alarm sirens from controller outputs
Example
In an Arduino-based greenhouse controller schematic, the 8-Channel Relay Board symbol's VCC pin connects to the 5 V Arduino power output and GND to Arduino ground; eight IN control pins connect to Arduino GPIO D2–D9; the eight NO contacts on the output side each switch a separate 120 V AC load (water pump, grow light, fan, heating mat) from shared neutral and hot bus bars.
Key facts
- The 8-Channel Relay Board has two logic-supply pins — VCC (typically 5 V DC) and GND — plus eight IN control inputs and eight sets of SPDT contacts (COM, NO, NC) per channel.
- Each channel is optocoupler-isolated (typically using PC817), preventing mains transients and ground-loop currents from reaching the microcontroller GPIO pins.
- Most 8-channel boards use active-LOW triggering: writing a GPIO LOW energises the relay and closes the NO contact — the GPIO must be set HIGH at startup to prevent all relays from energising during boot.
- Individual relay contact ratings are typically 10 A at 250 V AC / 30 V DC, sufficient for household appliances up to 2500 W per channel at 250 V.
- The VCC supply powers both the optocouplers and the relay coils; at ~75 mA per coil, all 8 relays energised simultaneously draw ~600 mA, which exceeds most Arduino 5 V pin current limits — an external 5 V supply is recommended.
- Some modules include a jumper to select whether the optocoupler is powered from the same VCC rail as the coils (JD-VCC jumper present) or from a separate isolated supply, providing higher isolation between the MCU logic ground and the relay coil ground.
- The 8-channel relay board occupies a standard breadboard or DIN-rail mount footprint and is widely compatible with Arduino, ESP32, ESP8266, and Raspberry Pi GPIO (with 3.3 V–5 V level shifting where needed).
Frequently asked questions
What does the 8-channel relay board symbol represent in a circuit diagram?
The 8-channel relay board symbol represents a pre-assembled module containing eight optocoupler-isolated SPDT relays on one PCB. It shows the VCC and GND logic supply pins and implies eight control inputs and eight sets of COM/NO/NC load-switching contacts for connecting to mains-voltage or high-current loads.
How do I connect an 8-channel relay board to an Arduino?
Connect the board's VCC to an external 5 V supply (not the Arduino 5 V pin, as the coil current can exceed the pin's rating), GND to common ground, and each IN1–IN8 control pin to an Arduino GPIO (D2–D9). Most boards use active-LOW logic, so setting a GPIO LOW energises the corresponding relay.
What voltage does an 8-channel relay board need?
Most 8-channel relay boards require 5 V DC on the VCC pin to power the optocouplers and relay coils. With all eight relays active, the board draws approximately 600 mA total, so a dedicated 5 V supply rated at least 1 A is recommended rather than drawing from a microcontroller's onboard regulator.
What loads can an 8-channel relay board switch?
Each channel's SPDT contacts are typically rated 10 A at 250 V AC and 10 A at 30 V DC, allowing control of household appliances, motors, lighting, and solenoids up to 2500 W at 250 V per channel. Inductive loads (motors, solenoids) require a snubber or flyback diode to suppress switching transients.
What does active-LOW triggering mean on a relay board?
Active-LOW triggering means the relay energises (NO contact closes) when the IN control pin is pulled to logic LOW (0 V). The relay de-energises when the pin is HIGH. This is the most common configuration on optocoupler-based relay boards; at power-up, GPIO pins that default HIGH will keep all relays de-energised.
Is a separate power supply required for the 8-channel relay board?
Yes, for reliable operation. Eight relay coils at ~75 mA each draw ~600 mA simultaneously; the Arduino 5 V pin is typically rated for only 500 mA total. A dedicated 5 V, 1 A or 2 A supply connected to VCC (with JD-VCC jumper removed for full isolation) prevents voltage droops that can cause microcontroller resets.
What is the JD-VCC jumper on an 8-channel relay board?
The JD-VCC jumper connects or separates the relay coil power supply (JD-VCC) from the optocoupler logic supply (VCC). With the jumper installed (default), both share the same supply. Removing the jumper and providing a separate 5 V to JD-VCC completely isolates the relay coil ground from the microcontroller ground, providing maximum electrical isolation.
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