Built-in project · Arduino Uno R3

16-channel relay board chaser on an Arduino Uno

Sixteen relays on one board from sixteen Uno pins: they click on one after another and off again, each lamp on the board following its relay. The whole build, an Arduino Uno and 2 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Beginner About 20 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $43. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

Active-low relay inputs
Driving many outputs from an array of pins
Why relay coils need their own supply
Setting the safe state before enabling an output
Using analog pins as digital outputs

Difficulty: Beginner
Time: About 20 minutes
Board: Arduino Uno R3
Parts: 3

How it works

A 16-channel relay board has one input per relay. The Uno drives all sixteen: IN1 to IN12 on pins 2 to 13, and IN13 to IN16 on A0 to A3. The relay coils run from a separate 12 V supply, and the grounds are joined.

The inputs are active LOW. A relay pulls in while its pin is LOW and drops out when the pin is HIGH.

The sketch clicks the relays on one after another with a 120 ms gap, then switches them off one after another in the same order. Then it repeats.

Parts list

3 parts, plus the jumper wires. They retail for about $43; on Mokxi they're free.

1 × Arduino Uno R3In Starter Kit, Claw Machine Kit, Vending Machine Kit.
1 × 16-channel relay board
1 × DC voltage source

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pin	Connects to
16-channel relay board pin GND	Arduino Uno R3 pin GND
16-channel relay board pin IN12	Arduino Uno R3 pin 13
16-channel relay board pin IN11	Arduino Uno R3 pin 12
16-channel relay board pin IN10	Arduino Uno R3 pin 11
16-channel relay board pin IN9	Arduino Uno R3 pin 10
16-channel relay board pin IN8	Arduino Uno R3 pin 9
16-channel relay board pin IN7	Arduino Uno R3 pin 8
16-channel relay board pin IN6	Arduino Uno R3 pin 7
16-channel relay board pin IN5	Arduino Uno R3 pin 6
16-channel relay board pin IN4	Arduino Uno R3 pin 5
16-channel relay board pin IN3	Arduino Uno R3 pin 4
16-channel relay board pin IN2	Arduino Uno R3 pin 3
16-channel relay board pin IN1	Arduino Uno R3 pin 2
DC voltage source pin V-	Arduino Uno R3 pin GND
16-channel relay board pin IN13	Arduino Uno R3 pin A0
16-channel relay board pin IN14	Arduino Uno R3 pin A1
16-channel relay board pin IN15	Arduino Uno R3 pin A2
16-channel relay board pin IN16	Arduino Uno R3 pin A3
DC voltage source pin V+	16-channel relay board pin VCC

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3

// A 16-channel relay board, chased from sixteen Uno pins.
//
//   IN1..IN12 -> pins 2 to 13     IN13..IN16 -> A0 to A3
//   VCC -> the 12 V supply (the board's relays have 12 V coils), GND -> GND
//
// The board's inputs are active LOW: a relay pulls in while its pin is LOW.

const uint8_t IN[16] = {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, A0, A1, A2, A3};

void setup() {
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], HIGH);  // off before the pin becomes an output
    pinMode(IN[i], OUTPUT);
  }
}

void loop() {
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], LOW);
    delay(120);
  }
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], HIGH);
    delay(120);
  }
}

Code walkthrough

The key functions of the sketch, and what each one does.

`setup` sketch.ino

For each of the 16 pins in IN, it writes HIGH first and only then makes the pin an output. HIGH means off for this board, so no relay clicks on at start-up.

void setup() {
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], HIGH);  // off before the pin becomes an output
    pinMode(IN[i], OUTPUT);
  }
}

`loop` sketch.ino

The first loop sets each pin LOW in turn, with 120 ms between, so the relays pull in one by one. The second loop sets each pin HIGH in turn, so they release one by one in the same order.

void loop() {
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], LOW);
    delay(120);
  }
  for (uint8_t i = 0; i < 16; i++) {
    digitalWrite(IN[i], HIGH);
    delay(120);
  }
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

Relays click on when you power up, or the pattern is backward This project
The board is active LOW. LOW turns a relay on and HIGH turns it off.
No relays click This project
Check that the relay board VCC is on the 12 V supply and its GND is joined to the Uno's GND.
Relays do not switch or the controller resets. 16-channel relay board
The coils need their own 12 V supply with enough current for many relays at once. Join the grounds of the controller and the board, and do not power coils from a microcontroller pin.
The logic is backward or relays click at start. 16-channel relay board
Check whether your board is active low or active high, and set pins to the off state before making them outputs. Switch relays on one at a time to limit the surge.
The circuit sees no voltage or the wrong amount. DC voltage source
Make sure V+ and V- go to the right nodes. The source floats, so tie one side to your circuit ground. Check the set voltage matches what your parts allow.
A part gets too much voltage. DC voltage source
Check the rating of every part before raising the supply. Use series resistors with LEDs, and do not connect the source straight across a short.
Upload fails or nothing runs after wiring pins 0 and 1 to a module. Arduino Uno R3
Pins 0 and 1 are the USB serial port. Move the module to other pins, or unplug it while uploading. Also check the right board is selected.
Pins misbehave or the board resets when you power motors or servos from it. Arduino Uno R3
Each I/O pin gives about 20 mA, 40 mA max. Power motors and servos from their own supply and join its ground to the Uno's GND.

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