Source: https://mokxi.com/parts/relay4 Updated: 2026-11-10 Part # Relay board, four channels Four relays in a row, active low, each on its own terminal. See every part - 18 pins - 4 settings - In 2 projects - In 1 lesson Drawn live by the editor's own code, at the size you see it. At a glance ## The Relay board, four channels in brief What it is, and every setting the properties panel gives it. The four-channel relay board puts four SRD relays in a row, with a GND, IN1 to IN4 and VCC header at one end and NO, COM and NC terminals for every channel. On an Uno, four digital pins drive the inputs and VCC goes to the 5 volt pin; on an ESP32 the 3.3 volt outputs still read as high against half of VCC, but VCC itself must be 5 volts, because under 4 volts Mokxi’s coils will not pull in. It is active low, so LOW switches a channel on and HIGH switches it off. Each channel has its own 10 millisecond pull-in and 5 millisecond release, with its red LED lit while energized. The 12 and 24 volt versions are a coil property: they need 9 and 18 volts to pull in, and an active-low 12 volt board driven straight from a 5 volt pin never turns off, so each input wants a transistor. Four coils draw about 280 milliamps, more than a USB-powered Uno likes, but coil current is not modeled, nor is the separate JD-VCC jumper or contact bounce. Family Outputs Pins 18 trigger low (or high) low (the default, as on nearly every board sold) or high. coil 5V (or 12V, 24V) 5V (the default), 12V or 24V, the SRD relay the board carries. pull_ms and pull_ms 10 release_ms 5 10 and 5 ms, the SRD-05VDC's times. Pins ## 18 pins Named as the editor names them, so a wire lands where you expect it. Pin Role What it does GND Input or output Ground. IN1 Input IN2 Input IN3 Input IN4 Input VCC Input or output The coils' supply, 5 V unless coil says 12 or 24 V. NO1 Input or output Each channel's changeover contact. COM1 Input or output Each channel's changeover contact. NC1 Input or output NO2 Input or output COM2 Input or output NC2 Input or output NO3 Input or output COM3 Input or output NC3 Input or output NO4 Input or output COM4 Input or output NC4 Input or output How to use it ## How to use a 4 channel relay module with an Arduino The relay board example drives IN1 to IN4 from pins 2 to 5, with the board’s VCC on 5 volts and its GND on the Uno’s ground. Each lamp is fed from its channel’s NO contact with every COM on the 5 volt rail, so a lamp lights only while its relay is pulled in, and the red LEDs on the board show which coils are energized. Swap the digitalWrite(pin, HIGH) and pinMode(pin, OUTPUT) lines in the example’s setup() and press Run: all four relays click on together at startup, which is what a real board does in the instant a pin sits low as an output. Written HIGH first, every channel stays off until the sketch asks for it. Part pin Board pin IN1 Pin 2 IN2 Pin 3 IN3 Pin 4 IN4 Pin 5 VCC 5 V GND GND COM1 to COM4 5 V rail NO1 to NO4 Each lamp’s resistor On an ESP32 the inputs can come straight from 3.3 volt pins, but VCC still needs 5 volts, or no coil pulls in. Click to open it in the editor This is the simulator itself, running here. Click anything to open it in the editor. Open this project in the editor Read the full tutorial How it is modeled ## What is true about the Relay board, four channels, here In its help article’s own words: what the simulation gets right, and where it stops. ### Modeled Everything the single module gets right, per channel: the 10 ms pull-in and 5 ms release, the arm touching neither contact while it travels, and no coil pulling in under 4 V of VCC on a 5 V board. Each channel's red LED lights while its coil is energized. ### Not modeled The optocoupler as a circuit: the input is a logic level against half of VCC, where a real one is an LED that a 3.3 V high on a 5 V board may still half light. The JD-VCC jumper that lets the coils run from their own supply is not brought out: VCC powers everything. Coil current (about 70 mA a channel, 280 mA for four and 560 mA for eight, more than a USB-powered board likes), contact bounce, and the 10 A contact rating. From Relay boards: two, four and eight channels, in full. Projects ## See the Relay board, four channels in a project 2 built-in projects use it. Each opens in the editor, already wired. Lessons ## Where it turns up in a lesson One lesson places it in its own circuit. ### Learn articles 1 lesson - Arduino Relay Module: Active LOW and Startup Clicks Boards ## The boards it is shown on This board has a built-in project with the Relay board, four channels in it. - Arduino Uno R3 More parts ## The rest of the bench Every one of these is drawn and simulated the same way. Full-size breadboard A real 0.1 inch grid with the rails and the center channel, 63 columns wide. Jumper wires Drag from any pin or hole to any other. Corners snap, and you can drag them. Power A supply rail at the voltage you choose. Ground The other end of every circuit. LED Lights when current flows. Five colors, and the brightness is what your eye would see. Filament lamp A tungsten bulb in a holder: an inrush when it is switched on cold, tens of milliseconds to warm up, and dim on half its voltage. Resistor Any value you like, with the color bands drawn to match. ESP32-C3-DevKitM-1 A RISC-V board that runs your firmware at 160 MHz on the real memory map. Pushbutton A 12 mm tactile switch. Hold it while the simulation runs. Raspberry Pi Pico The RP2040 board on our own Cortex-M0+ core. Pick a program and press Run. Raspberry Pi Pico W Raspberry Pi Pico W (RP2040). The same board and the same pinout as a Pico, with the CYW43439 on it. The WiFi is simulated (no radio; real internet through the Cloud panel’s switch), and the on-board LED, which hangs off that chip rather than off GP25, is driven through the same path. Everything else is the Pico. Raspberry Pi 4 Model B Raspberry Pi 4 Model B: the 40-pin header, driven by main.py with RPi.GPIO or gpiozero. See every part ## Wire up the Relay board, four channels Open the editor and push it into the breadboard. It is free, and it runs on your own machine. Start building free Open the editor