Source: https://mokxi.com/parts/relay Updated: 2026-09-27 Part # Relay module A logic level on one side, something that has nothing to do with logic on the other. or see every part - 6 pins - 3 properties Drawn live by the editor's own code, at the size you see it. Reference ## Every one of its 6 pins Pin Role What it does GND Input or output Coil return. IN Input Control input; a logic level against half the supply. VCC Input or output Coil supply. NO Input or output Normally open: where COM moves to when energized. COM Input or output The arm. NC Input or output Normally closed: where COM rests when not energized. This part ## What it does A relay module lets a microcontroller pin switch something that has nothing to do with it. A logic level on IN energizes a coil, the coil pulls a metal arm across, and the arm moves from the normally closed contact to the normally open one; COM is the arm. The board’s 5 volts and whatever the contacts switch share nothing at all. Mokxi’s relay is slow in the way a real one is: 10 milliseconds to pull in and 5 to drop out, with neither contact made while the arm is traveling, so a sketch that toggles it very fast gets nothing, here and on the bench. Below 4 volts on the coil supply it does not pull in whatever IN says. The trigger property is high or low, because most cheap blue relay boards switch when IN is pulled to ground and a sketch written for the other kind runs backwards. How to use it ## How to use a relay module with an Arduino VCC to 5 V, GND to GND and IN to a digital pin. On the other side, COM is the moving arm: wire it to the supply for the thing you are switching, and wire that thing to NO so it is off until the relay pulls in. Then digitalWrite() the IN pin. Check which way your board triggers. Most cheap blue relay boards are active low and switch when IN goes to ground; set the part’s trigger property to match the one you have. Never switch a relay faster than it can move: it takes about 10 milliseconds to pull in. Part pin Board pin IN Pin 13 VCC 5 V GND GND COM and NO The supply and the load 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 Reference ## The 3 properties you can set Property Default What it means trigger high (or low) high (the default here, matching a bare coil and transistor) or low (the cheap blue boards are nearly all active-low, because their opto-coupler is wired to the rail). pull_ms 10 10 and 5 ms, the SRD-05VDC datasheet numbers. release_ms 5 10 and 5 ms, the SRD-05VDC datasheet numbers. How it is modeled ## What is true about the Relay module, here ### What is modeled A relay is slow, and the timing is the part people get wrong: 10 ms to pull in and 5 ms to drop out, matching the datasheet. During that travel neither contact is made (the arm is genuinely in the air, which is "break before make"), so a sketch toggling a relay at a kilohertz does nothing at all here either, exactly as it does nothing on the bench. Below 4 V across VCC/GND the coil cannot pull the arm over at all, whatever IN says, which is the failure everyone meets running a relay off a board's regulator with a motor on the same rail. ### Not modeled Coil current (about 70 mA, which matters for the transistor driving it), contact bounce on the way in, the arc across an inductive load, and the flyback diode, which is already on the module, and is why nothing here needs one added. The coil is not a coil: VCC/GND are read only to decide whether the supply is above 4 V, so the control side draws nothing at all and there is no inductance, no pull-in current, no hold current and no way to find out that a board's regulator cannot run one. IN is a logic level against half the supply rather than the opto-coupler's LED. pull_ms and release_ms are exact and identical on every operation, where a real relay's travel time varies with coil voltage and temperature and gets longer as it ages. The contacts are ideal metal with no rating, no resistance drift and no welding. From Relay module (SRD-05VDC style), in full. Projects ## See the Relay module in a project Shown here on: Arduino Uno R3 Learn ## Where it turns up in a lesson ### In a learn article - Arduino 4x4 Keypad Code Lock with a Relay 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. 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, no real internet), 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. STM32F411 Black Pill The Black Pill on our own Cortex-M4 core. Pick a program and press Run. BBC micro:bit V2 The nRF52833 board with its 5x5 LED matrix, buttons A and B and a speaker. Wire the rings to a breadboard. See every part ## Wire up the Relay module Open the editor and push it into the breadboard. It is free, and it runs on your own machine. Start building Open the editor