Single Channel Relay with ESP8266 (Step-by-Step Guide)

Introduction

A relay module is an electrically operated switch that allows the ESP8266 to control high-voltage devices like lamps, fans, and home appliances using its low-voltage GPIO pins. Since ESP8266 works at 3.3V logic, it cannot directly switch high-power loads. That’s why we use a single channel relay module, which acts as a bridge between the ESP8266 and AC/DC appliances.

Relays are widely used in home automation systems, industrial control projects, and IoT applications. By connecting a relay to ESP8266, you can control electrical devices remotely via Wi-Fi, mobile apps, or IoT dashboards.

In this tutorial, we will connect a single channel relay to the ESP8266 and control it with simple Arduino code.

Components Required

• ESP8266 (NodeMCU / Wemos D1 Mini)
• Single Channel Relay Module (5V)
• Jumper Wires
• Breadboard
• USB Cable

⚠️ Safety Note: Be very careful when connecting AC appliances to a relay. If you’re a beginner, practice with a low-voltage DC load (like an LED strip or 12V bulb) before working with AC mains.

Circuit Diagram (Relay → ESP8266)

• Relay IN → D1 (GPIO5)
• Relay VCC → 3.3V / 5V (depending on relay type, many modules support 5V)
• Relay GND → GND
• Common (COM) and Normally Open (NO) → Connect to your appliance

Arduino Code for Relay Control

// Single Channel Relay with ESP8266
int relayPin = D1;  // Relay connected to GPIO5 (D1)

void setup() {
  pinMode(relayPin, OUTPUT);   // Set relay pin as output
  digitalWrite(relayPin, HIGH); // Keep relay OFF initially
  Serial.begin(115200);
}

void loop() {
  // Turn relay ON
  digitalWrite(relayPin, LOW);  // Active LOW relay
  Serial.println("Relay ON");
  delay(2000);

  // Turn relay OFF
  digitalWrite(relayPin, HIGH);
  Serial.println("Relay OFF");
  delay(2000);
}

Step-by-Step Code Explanation

Relay Pin Declaration

int relayPin = D1;

We connect the relay input pin to D1 (GPIO5) of ESP8266.

Setup Function

pinMode(relayPin, OUTPUT);
digitalWrite(relayPin, HIGH);

The relay pin is set as an output.

By default, the relay is kept OFF (HIGH), since most relay modules are active LOW.

Loop Function

digitalWrite(relayPin, LOW); 
delay(2000);
digitalWrite(relayPin, HIGH);
delay(2000);

Relay turns ON for 2 seconds and then OFF for 2 seconds repeatedly.

When ON, it connects COM → NO, allowing current to flow to the load.

Output

• The relay will click ON and OFF every 2 seconds.
• If connected to a bulb or LED strip, it will blink accordingly.

Real-Life Applications

1. Home Automation:
   Control lights, fans, and appliances using Wi-Fi-enabled ESP8266.
2. Smart Plugs:
   Create your own IoT-enabled plug that can be switched ON/OFF remotely.
3. Irrigation Systems:
   Turn ON water pumps automatically based on soil moisture levels.
4. Industrial Automation:
   Switch heavy machinery or motors with safety isolation.
5. Security Systems:
   Use relays to activate alarms, sirens, or emergency cut-off switches.

Troubleshooting

• Relay Not Switching ON:
  • Check if your relay module is active LOW or active HIGH. Adjust your code accordingly.
  • Ensure the relay VCC is supplied with enough voltage (3.3V or 5V depending on module).
• Clicking Sound But Load Not Working:
  • Check wiring of COM and NO pins.
  • Make sure your load is connected properly and powered correctly.
• ESP8266 Keeps Restarting:
  • Some relays draw high current when switching. Use a separate 5V power supply if needed.
• Relay Works Opposite (ON when OFF, OFF when ON):
  • Invert the logic in code (HIGH for ON and LOW for OFF).
• Danger with AC Loads:
  • Always use relays with opto-isolators for AC appliances.
  • Beginners should test only with low-voltage DC loads first.