Relay Module Control

Control high-power devices remotely using relay modules with ESP32

What is a Relay?

A relay is an electrically operated switch that allows low-voltage circuits (like ESP32 at 3.3V) to control high-voltage/high-current circuits (like 220V AC appliances). It provides electrical isolation between control and power circuits for safety.

Relay Module Specifications

Control Side (Low Voltage)

Operating Voltage: 3.3V - 5V DC
Trigger Current: 5-20 mA
Trigger Type: Active Low or High
Input: Digital signal from MCU
Response Time: 5-10 ms

Power Side (High Voltage)

Max AC Voltage: 250V AC
Max DC Voltage: 30V DC
Max Current: 10A (typical)
Max Power: 2500W (10A @ 250V)
Contacts: NO, NC, COM

Current Rating

Always check your specific relay module's current rating. Common ratings are 5A and 10A. Never exceed the rated current - add a safety margin (use only 80% of rated capacity for continuous operation).

Pin Configuration

Control Pins (Low Voltage Side)

Pin	Function	ESP32 Connection
VCC	Power supply (3.3V or 5V)	3.3V or 5V pin
GND	Ground	GND pin
IN1	Control signal for relay 1	GPIO 25
IN2	Control signal for relay 2	GPIO 26

Power Terminals (High Voltage Side)

Terminal	Name	Description
1	COM	Common - always connected to power source
2	NO	Normally Open - closed when relay activated
3	NC	Normally Closed - opens when relay activated

Note: For most applications, use COM and NO terminals. The load is OFF when relay is inactive, ON when relay is activated.

Active Low vs Active High

Active Low (Common)

Relay activates when signal is LOW (0V):

digitalWrite(pin, LOW) → Relay ON
digitalWrite(pin, HIGH) → Relay OFF
Most relay modules use this

Active High (Less Common)

Relay activates when signal is HIGH (3.3V):

digitalWrite(pin, HIGH) → Relay ON
digitalWrite(pin, LOW) → Relay OFF
Check your module's documentation

How to Identify

Most Chinese relay modules have LED indicators. Connect power and leave IN pin floating or connected to VCC - if LED is ON, it's Active Low. If LED is OFF, it's Active High.

Basic Control Code

relay_basic.inocpp

// Define relay pins
#define RELAY1_PIN 25
#define RELAY2_PIN 26

void setup() {
  Serial.begin(115200);
  
  // Initialize relay pins as outputs
  pinMode(RELAY1_PIN, OUTPUT);
  pinMode(RELAY2_PIN, OUTPUT);
  
  // Initial state - all relays OFF
  // For Active Low modules:
  digitalWrite(RELAY1_PIN, HIGH);  // OFF
  digitalWrite(RELAY2_PIN, HIGH);  // OFF
  
  Serial.println("Relay control initialized");
}

void loop() {
  // Turn relay 1 ON
  Serial.println("Relay 1 ON");
  digitalWrite(RELAY1_PIN, LOW);   // For Active Low
  delay(3000);
  
  // Turn relay 1 OFF
  Serial.println("Relay 1 OFF");
  digitalWrite(RELAY1_PIN, HIGH);  // For Active Low
  delay(3000);
  
  // Turn relay 2 ON
  Serial.println("Relay 2 ON");
  digitalWrite(RELAY2_PIN, LOW);
  delay(3000);
  
  // Turn relay 2 OFF
  Serial.println("Relay 2 OFF");
  digitalWrite(RELAY2_PIN, HIGH);
  delay(3000);
  
  // Both relays ON
  Serial.println("Both relays ON");
  digitalWrite(RELAY1_PIN, LOW);
  digitalWrite(RELAY2_PIN, LOW);
  delay(3000);
  
  // Both relays OFF
  Serial.println("Both relays OFF");
  digitalWrite(RELAY1_PIN, HIGH);
  digitalWrite(RELAY2_PIN, HIGH);
  delay(3000);
}

Control via WiFi Commands

relay_wifi_control.inocpp

#include <WiFi.h>
#include <WebServer.h>

const char* ssid = "YOUR_WIFI_SSID";
const char* password = "YOUR_WIFI_PASSWORD";

#define RELAY1_PIN 25
#define RELAY2_PIN 26

WebServer server(80);

// Relay states
bool relay1State = false;
bool relay2State = false;

void setup() {
  Serial.begin(115200);
  
  // Initialize relays
  pinMode(RELAY1_PIN, OUTPUT);
  pinMode(RELAY2_PIN, OUTPUT);
  digitalWrite(RELAY1_PIN, HIGH);  // OFF
  digitalWrite(RELAY2_PIN, HIGH);  // OFF
  
  // Connect to WiFi
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  
  Serial.println("\nConnected to WiFi");
  Serial.print("IP: ");
  Serial.println(WiFi.localIP());
  
  // Setup web server routes
  server.on("/", handleRoot);
  server.on("/relay1/on", []() { controlRelay(1, true); });
  server.on("/relay1/off", []() { controlRelay(1, false); });
  server.on("/relay2/on", []() { controlRelay(2, true); });
  server.on("/relay2/off", []() { controlRelay(2, false); });
  server.on("/status", handleStatus);
  
  server.begin();
  Serial.println("Web server started");
}

void loop() {
  server.handleClient();
}

void controlRelay(int relayNum, bool state) {
  int pin = (relayNum == 1) ? RELAY1_PIN : RELAY2_PIN;
  
  // Active Low: LOW = ON, HIGH = OFF
  digitalWrite(pin, state ? LOW : HIGH);
  
  if (relayNum == 1) {
    relay1State = state;
  } else {
    relay2State = state;
  }
  
  String response = "Relay " + String(relayNum) + " turned " + (state ? "ON" : "OFF");
  server.send(200, "text/plain", response);
  
  Serial.println(response);
}

void handleRoot() {
  String html = "<html><body>";
  html += "<h1>Gridova Relay Control</h1>";
  html += "<p>Relay 1: " + String(relay1State ? "ON" : "OFF") + "</p>";
  html += "<a href='/relay1/on'><button>Turn ON</button></a> ";
  html += "<a href='/relay1/off'><button>Turn OFF</button></a><br><br>";
  html += "<p>Relay 2: " + String(relay2State ? "ON" : "OFF") + "</p>";
  html += "<a href='/relay2/on'><button>Turn ON</button></a> ";
  html += "<a href='/relay2/off'><button>Turn OFF</button></a>";
  html += "</body></html>";
  
  server.send(200, "text/html", html);
}

void handleStatus() {
  String json = "{\"relay1\":" + String(relay1State ? "true" : "false");
  json += ",\"relay2\":" + String(relay2State ? "true" : "false") + "}";
  
  server.send(200, "application/json", json);
}

Access the web interface at: http://ESP32_IP_ADDRESS/

Integration with Gridova API

gridova_relay_integration.inocpp

#include <WiFi.h>
#include <HTTPClient.h>
#include <ArduinoJson.h>

const char* serverUrl = "https://api.gridova.com/relay/control";
const char* deviceId = "ROOM_001";
unsigned long lastCheckTime = 0;
const unsigned long checkInterval = 5000; // Check every 5 seconds

#define RELAY1_PIN 25
#define RELAY2_PIN 26

void setup() {
  Serial.begin(115200);
  
  pinMode(RELAY1_PIN, OUTPUT);
  pinMode(RELAY2_PIN, OUTPUT);
  digitalWrite(RELAY1_PIN, HIGH);
  digitalWrite(RELAY2_PIN, HIGH);
  
  // Connect to WiFi
  WiFi.begin("SSID", "PASSWORD");
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
  
  Serial.println("Connected!");
}

void loop() {
  unsigned long currentTime = millis();
  
  // Check server for relay commands
  if (currentTime - lastCheckTime >= checkInterval) {
    checkRelayCommands();
    lastCheckTime = currentTime;
  }
}

void checkRelayCommands() {
  if (WiFi.status() != WL_CONNECTED) return;
  
  HTTPClient http;
  String url = String(serverUrl) + "?device_id=" + deviceId;
  
  http.begin(url);
  int httpCode = http.GET();
  
  if (httpCode == 200) {
    String payload = http.getString();
    
    // Parse JSON response
    StaticJsonDocument<256> doc;
    DeserializationError error = deserializeJson(doc, payload);
    
    if (!error) {
      bool relay1 = doc["relay1"];
      bool relay2 = doc["relay2"];
      
      // Update relay states
      digitalWrite(RELAY1_PIN, relay1 ? LOW : HIGH);
      digitalWrite(RELAY2_PIN, relay2 ? LOW : HIGH);
      
      Serial.printf("Updated: Relay1=%s, Relay2=%s\n", 
                    relay1 ? "ON" : "OFF", 
                    relay2 ? "ON" : "OFF");
    }
  }
  
  http.end();
}

// Function to report relay state to server
void reportRelayState() {
  HTTPClient http;
  http.begin(serverUrl);
  http.addHeader("Content-Type", "application/json");
  
  StaticJsonDocument<256> doc;
  doc["device_id"] = deviceId;
  doc["relay1"] = (digitalRead(RELAY1_PIN) == LOW);
  doc["relay2"] = (digitalRead(RELAY2_PIN) == LOW);
  doc["timestamp"] = millis();
  
  String jsonString;
  serializeJson(doc, jsonString);
  
  int httpCode = http.POST(jsonString);
  Serial.printf("Report sent: %d\n", httpCode);
  
  http.end();
}

High Voltage Wiring

Electrical Safety Warning

This section involves working with mains voltage (220V AC). Only qualified personnel should perform these connections. Improper wiring can cause fire, electric shock, or death.

Wiring Diagram Explanation

AC Mains (220V) → Circuit Breaker → Relay COM
Relay NO → Load (Appliance) → Neutral

Step-by-step:

Disconnect all power sources
Connect Live wire from breaker to relay COM terminal
Connect relay NO terminal to appliance Live wire
Connect appliance Neutral directly to mains Neutral
Connect Ground/Earth to appliance chassis
Secure all connections with proper wire nuts or terminals
Install in proper electrical enclosure
Test with multimeter before applying power

Safety Considerations

Current Rating & Load

Calculate total load current: Power (W) ÷ Voltage (V) = Current (A)
Use relay rated for at least 1.5x your load current
For inductive loads (motors, compressors), use 2x rating
Add heat sink for continuous high-current operation

Protection Devices

Always use circuit breaker or fuse (rated for load + 20%)
Add surge protector for sensitive electronics
Use GFCI/RCD for wet locations
Consider adding snubber circuit for inductive loads

Enclosure & Installation

Use IP-rated electrical box (IP54 minimum for indoor)
Ensure proper ventilation to prevent overheating
Label all wires clearly
Use appropriate wire gauge (14 AWG for 15A, 12 AWG for 20A)
Maintain proper clearance between terminals

Firmware Safety Features

Implement watchdog timer to reset if system hangs
Add maximum ON time limit (prevent stuck-on relay)
Use failsafe default state (OFF on boot)
Log all relay state changes with timestamp
Add manual override button for emergency shutoff

Advanced: Failsafe Code

relay_failsafe.inocpp

#include <esp_task_wdt.h>

#define RELAY1_PIN 25
#define MAX_ON_TIME 3600000  // 1 hour in milliseconds
#define WDT_TIMEOUT 10       // 10 seconds

unsigned long relay1OnTime = 0;
bool relay1Active = false;

void setup() {
  Serial.begin(115200);
  
  pinMode(RELAY1_PIN, OUTPUT);
  digitalWrite(RELAY1_PIN, HIGH);  // OFF by default (failsafe)
  
  // Setup watchdog timer
  esp_task_wdt_init(WDT_TIMEOUT, true);
  esp_task_wdt_add(NULL);
  
  Serial.println("Failsafe relay control initialized");
}

void loop() {
  // Reset watchdog
  esp_task_wdt_reset();
  
  // Check maximum ON time
  if (relay1Active) {
    if (millis() - relay1OnTime > MAX_ON_TIME) {
      Serial.println("Max ON time reached - turning OFF");
      setRelay1(false);
    }
  }
  
  // Your control logic here
  delay(100);
}

void setRelay1(bool state) {
  digitalWrite(RELAY1_PIN, state ? LOW : HIGH);
  relay1Active = state;
  
  if (state) {
    relay1OnTime = millis();
    Serial.println("Relay 1 ON");
  } else {
    Serial.println("Relay 1 OFF");
  }
  
  // Log to EEPROM or send to server
  logRelayState(1, state);
}

void logRelayState(int relay, bool state) {
  // Save to EEPROM or send to server
  Serial.printf("LOG: Relay %d = %s at %lu\n", 
                relay, state ? "ON" : "OFF", millis());
}

Troubleshooting

Relay Clicks But Load Doesn't Turn On

Check if using correct terminals (COM and NO, not NC)
Verify load is plugged in and functional
Test with multimeter across NO-COM when activated
Check wire connections are secure

Relay Doesn't Click

Check if relay module has power (VCC and GND)
Verify GPIO pin is configured as OUTPUT
Test if LED indicator turns on
Check if using correct trigger logic (Active Low/High)
Measure voltage on IN pin when triggered

Relay Gets Hot

Load current may exceed relay rating
Check for loose connections causing resistance
Ensure proper ventilation around relay
Consider using contactor for high-power loads

Random Triggering

Add pull-up/pull-down resistor on control pin
Ensure stable power supply
Check for electromagnetic interference
Use shielded cables for control signals
Add 0.1µF capacitor between VCC and GND

Next Steps

Complete Wiring Diagram

See how relay integrates with ESP32, PZEM sensor, and AC power

WebSocket Real-time Control

Implement instant relay control via WebSocket connection

Mobile App Integration

Control relays from Gridova Flutter mobile app

gettingStarted

hardware

api

configuration