ESP32 Relay Control

Control a relay (light, pump, fan, door lock) from your SiliconWit.io dashboard while also reading sensor data — a complete bidirectional IoT example.

What You’ll Build

• ESP32 reads a DHT22 temperature/humidity sensor and sends data to the cloud
• Dashboard shows live readings and an on/off toggle for a relay
• Clicking the toggle sends a command over MQTT; the ESP32 switches the relay

┌─────────────┐         MQTT (TLS)         ┌──────────────────┐
│   ESP32      │ ◄──────────────────────── │  SiliconWit.io   │
│  + DHT22     │  commands topic            │    Dashboard     │
│  + Relay     │ ────────────────────────► │   (toggle relay) │
│              │  telemetry topic           │                  │
└─────────────┘                            └──────────────────┘

Requirements

Hardware

• ESP32 development board
• 1-channel relay module (5V or 3.3V, with optocoupler)
• DHT22 temperature/humidity sensor
• Jumper wires, breadboard
• USB cable for programming

Software

• Arduino IDE with ESP32 board support (setup guide)
• Libraries: PubSubClient, ArduinoJson, DHT sensor library

SiliconWit.io Setup

• A registered device (see Create Your Device below)

Wiring

ESP32 Pin	Component
GPIO 4	DHT22 data pin (with 10K pull-up to 3.3V)
GPIO 26	Relay IN (signal)
3.3V	DHT22 VCC
5V (VIN)	Relay VCC
GND	DHT22 GND, Relay GND

Caution

Most relay modules are active LOW — sending LOW turns the relay on. Check your module’s documentation.

Full Code

#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <PubSubClient.h>
#include <ArduinoJson.h>
#include <DHT.h>

// ─── Configuration ───────────────────────────────
const char* WIFI_SSID     = "YOUR_WIFI_SSID";
const char* WIFI_PASS     = "YOUR_WIFI_PASSWORD";

const char* MQTT_BROKER   = "mqtt.siliconwit.io";
const int   MQTT_PORT     = 8883;
const char* DEVICE_ID     = "SWD-XXXXXX";     // from dashboard
const char* ACCESS_TOKEN  = "your-access-token";   // from dashboard

// ─── Pins ────────────────────────────────────────
#define DHT_PIN   4
#define RELAY_PIN 26
#define DHT_TYPE  DHT22

// ─── Globals ─────────────────────────────────────
DHT dht(DHT_PIN, DHT_TYPE);
WiFiClientSecure espClient;
PubSubClient mqtt(espClient);

String telemetryTopic;
String commandTopic;

unsigned long lastSend = 0;
const unsigned long SEND_INTERVAL = 15000; // 15 seconds

// ─── MQTT command handler ────────────────────────
void onMessage(char* topic, byte* payload, unsigned int length) {
  // Parse JSON command
  JsonDocument doc;
  DeserializationError err = deserializeJson(doc, payload, length);
  if (err) {
    Serial.print("JSON parse error: ");
    Serial.println(err.c_str());
    return;
  }

  const char* command = doc["command"];
  if (!command) return;

  Serial.print("Command received: ");
  Serial.println(command);

  // Handle relay command
  if (strcmp(command, "relay") == 0) {
    bool state = doc["value"] | false;
    digitalWrite(RELAY_PIN, state ? LOW : HIGH); // active LOW relay
    Serial.print("Relay → ");
    Serial.println(state ? "ON" : "OFF");
  }
}

// ─── WiFi ────────────────────────────────────────
void setupWifi() {
  Serial.print("Connecting to WiFi");
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.print("\nConnected — IP: ");
  Serial.println(WiFi.localIP());
}

// ─── MQTT connect + subscribe ────────────────────
void connectMqtt() {
  while (!mqtt.connected()) {
    Serial.print("Connecting to MQTT...");
    if (mqtt.connect(DEVICE_ID, DEVICE_ID, ACCESS_TOKEN)) {
      Serial.println("connected");
      // Subscribe to commands topic
      mqtt.subscribe(commandTopic.c_str(), 1);
      Serial.print("Subscribed to: ");
      Serial.println(commandTopic);
    } else {
      Serial.print("failed (rc=");
      Serial.print(mqtt.state());
      Serial.println(") retrying in 5s");
      delay(5000);
    }
  }
}

// ─── Setup ───────────────────────────────────────
void setup() {
  Serial.begin(115200);

  // Build topic strings
  telemetryTopic = "d/" + String(DEVICE_ID) + "/t";
  commandTopic   = "d/" + String(DEVICE_ID) + "/c";

  // Relay pin — start OFF (HIGH for active-low)
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, HIGH);

  dht.begin();
  setupWifi();

  espClient.setInsecure(); // Use setCACert() in production
  mqtt.setServer(MQTT_BROKER, MQTT_PORT);
  mqtt.setCallback(onMessage);
}

// ─── Loop ────────────────────────────────────────
void loop() {
  if (!mqtt.connected()) {
    connectMqtt();
  }
  mqtt.loop();

  // Send sensor data periodically
  if (millis() - lastSend >= SEND_INTERVAL) {
    lastSend = millis();

    float temp = dht.readTemperature();
    float hum  = dht.readHumidity();

    if (!isnan(temp) && !isnan(hum)) {
      JsonDocument doc;
      doc["temperature"] = round(temp * 10) / 10.0;
      doc["humidity"]    = round(hum * 10) / 10.0;

      char buf[128];
      serializeJson(doc, buf);
      mqtt.publish(telemetryTopic.c_str(), buf);

      Serial.print("Sent: ");
      Serial.println(buf);
    }
  }
}

Step-by-Step Walkthrough

Create Your Device on SiliconWit.io

1. Go to Dashboard > Devices > Add Device

2. Fill in the device details:

   Field	Value	Why
   Name	ESP32 Relay	Descriptive name shown on your dashboard
   Type	Controller	The device sends sensor data and receives commands
   Data Direction	Bidirectional	Sends telemetry and receives relay toggle commands
   Connectivity	WiFi + MQTT	Uses MQTT over TLS for real-time communication
   Data Interval	15 seconds	How often the device sends readings

3. Configure Data Fields - define the telemetry fields the device will send:

   [
     { "name": "temperature", "label": "Temperature", "unit": "°C" },
     { "name": "humidity", "label": "Humidity", "unit": "%" }
   ]

   Tip

   To import fields: click Import in the Data Fields section, paste the JSON above, click Validate, then Import. See the Field Schema guide for nested fields and roles.

4. Configure Commands - add a toggle command so you can control the relay from the dashboard:

   [
     { "name": "relay", "label": "Light Relay", "type": "toggle" }
   ]

5. Click “Create Device” and copy the Device ID and Access Token.

Caution

Save your Access Token immediately! It is only shown once. Copy it somewhere safe before closing the dialog.

2. Upload the code

1. Replace YOUR_WIFI_SSID, YOUR_WIFI_PASSWORD, SWD-XXXXXX, and your-access-token in the code
2. Upload to your ESP32
3. Open Serial Monitor at 115200 baud — you should see WiFi connect, then MQTT connect

3. Test from the dashboard

1. Open your device page — you’ll see temperature readings arriving
2. In the Control Panel sidebar, click ON next to “Light Relay”
3. The relay should click and the connected load should turn on
4. Click OFF to turn it back off
5. Check the Command History section to see the sent commands

How It Works

1. The ESP32 subscribes to d/{id}/c on connect
2. When you press a toggle in the dashboard, the server publishes {"command":"relay","value":true} to that topic via the MQTT broker
3. The onMessage callback fires on the ESP32, parses the JSON, and sets the relay GPIO
4. Meanwhile, every 15 seconds the ESP32 publishes sensor data to d/{id}/t

Adding More Controls

You can add multiple commands to one device. For example, add a second relay or a PWM-controlled fan:

Second relay (toggle)

Add another command in device settings: name relay2, label Fan Relay, type Toggle.

Then extend the callback:

if (strcmp(command, "relay2") == 0) {
  bool state = doc["value"] | false;
  digitalWrite(RELAY2_PIN, state ? LOW : HIGH);
}

Fan speed (slider)

Add a command: name fan_speed, label Fan Speed, type Slider, min 0, max 255.

if (strcmp(command, "fan_speed") == 0) {
  int speed = doc["value"] | 0;
  analogWrite(FAN_PIN, constrain(speed, 0, 255));
}

Custom JSON command

For advanced use cases, add a command with type JSON (Custom). This lets you send any JSON payload from the dashboard — useful for multi-field configuration, schedules, or batch operations.

Add a command in device settings: name config, label Device Config, type JSON.

From the dashboard, you can type any valid JSON in the text area, for example:

{"mode": "auto", "threshold": 30, "interval": 10}

Then parse the full value object on the ESP32:

if (strcmp(command, "config") == 0) {
  // The "value" field contains whatever JSON you sent
  JsonObject cfg = doc["value"];
  const char* mode = cfg["mode"] | "manual";
  int threshold    = cfg["threshold"] | 25;
  int interval     = cfg["interval"] | 15;

  Serial.print("Mode: "); Serial.println(mode);
  Serial.print("Threshold: "); Serial.println(threshold);
  Serial.print("Interval: "); Serial.println(interval);

  // Apply settings to your device logic...
}

Tip

JSON commands are especially useful for sending configuration that has multiple fields at once, rather than creating separate slider/toggle commands for each setting.

Troubleshooting

Problem	Solution
Relay doesn’t click	Check wiring, verify relay VCC gets 5V, try inverting HIGH/LOW
Commands not received	Ensure device direction is Bidirectional, check Serial Monitor for subscribe confirmation
Sensor reads NaN	Check DHT22 wiring, add 10K pull-up resistor on data pin
MQTT disconnects	Increase mqtt.setKeepAlive(60), check WiFi stability
Relay clicks but load doesn’t switch	Verify load wiring to relay’s NO/COM terminals

Next Steps

• ESP32 Setup — Basic sensor-only setup and TLS configuration
• Topics & Payloads — Full MQTT message format reference
• Dashboard Alerts — Trigger alerts when temperature exceeds a threshold