Tutorial 22: Understanding Positive vs Negative Logic for Relays

Introduction to Relay Logic

When controlling relays with a microcontroller, the relationship between the GPIO pin state (HIGH or LOW) and the relay's ON/OFF state depends on the relay module's design. This is called relay logic.

Why Do Relay Modules Have Different Logic?

The logic type depends on the relay module's internal circuit:

• Optocoupler Design - Some modules use an optocoupler that triggers on HIGH, others on LOW
• Transistor Type - NPN transistors typically use Positive Logic, PNP transistors use Negative Logic
• Manufacturer Choice - Different manufacturers use different designs
• Historical Reasons - Some modules were designed for specific microcontrollers

How to Identify Your Relay Module's Logic

Method 1: The "Click" Test (Easiest)

1. Connect relay module VCC to 5V and GND to GND
2. Connect a GPIO pin to the relay's IN pin
3. Upload a simple sketch that toggles the pin HIGH and LOW every second
4. Listen for the relay clicking sound
5. Note which state (HIGH or LOW) makes the relay click

// Relay Logic Test Sketch
// Upload this to identify your relay's logic

#define RELAY_PIN D1  // Change to your GPIO

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println("Relay Logic Test");
  Serial.println("Listen for clicks every second");
}

void loop() {
  Serial.println("HIGH (3.3V/5V) - Relay should be OFF if Positive Logic, ON if Negative");
  digitalWrite(RELAY_PIN, HIGH);
  delay(1000);
  
  Serial.println("LOW (0V) - Relay should be ON if Positive Logic, OFF if Negative");
  digitalWrite(RELAY_PIN, LOW);
  delay(1000);
}

Method 2: Multimeter Test

1. Set your multimeter to continuity mode (beeper)
2. Connect the relay module as usual (VCC, GND, IN to GPIO)
3. Set GPIO to HIGH, check if COM and NO terminals are connected
4. Set GPIO to LOW, check again
5. The state that closes the circuit is the relay's ON state

Method 3: Check the Relay Module PCB

• Look for an optocoupler (typically a 4-pin IC like PC817)
• If the optocoupler LED anode connects to VCC → Negative Logic (active LOW)
• If the optocoupler LED anode connects to IN pin → Positive Logic (active HIGH)
• Note: This requires reading the schematic or tracing PCB traces

OceanRemote Relay Logic Configuration

OceanRemote allows you to set the logic for each relay individually. Here's how:

Step 1: During Device Creation

When generating firmware from the "Add New Device" page, you can select the logic for each relay:

Step 2: Changing Logic After Device Creation

1. Go to your OceanRemote dashboard
2. Find your device card
3. Click "DETAILS" button
4. In the device detail page, you can edit relay names and logic
5. Changes take effect immediately (no need to re-flash)

Understanding the Dashboard Display

OceanRemote shows the logic type for each relay in the dashboard:

• 🔺 (Triangle up) - Positive Logic (HIGH = ON)
• 🔻 (Triangle down) - Negative Logic (LOW = ON)

Example Dashboard Display:
┌─────────────────────────────────────┐
│  PIN MAPPING                        │
│  ┌─────┐ ┌─────┐ ┌─────┐ ┌─────┐   │
│  │ R1  │ │ R2  │ │ R3  │ │ R4  │   │
│  │ 🔺  │ │ 🔻  │ │ 🔺  │ │ 🔻  │   │
│  │Light│ │ Fan │ │Pump │ │Heat │   │
│  └─────┘ └─────┘ └─────┘ └─────┘   │
│                                     │
│  R1: Positive Logic (🔺)            │
│  R2: Negative Logic (🔻)            │
└─────────────────────────────────────┘

Practical Examples

Example 1: You have a 5V 4-channel relay module from Amazon

• Likely Logic: Negative Logic (Active LOW)
• OceanRemote Setting: Negative Logic (🔻)
• Result: Clicking "ON" in dashboard sends LOW to GPIO → relay clicks ON

Example 2: You have an industrial PLC relay module

• Likely Logic: Positive Logic (Active HIGH)
• OceanRemote Setting: Positive Logic (🔺)
• Result: Clicking "ON" in dashboard sends HIGH to GPIO → relay clicks ON

Example 3: Mixed logic (different relays on same module)

• Some relay modules have different logic for different channels
• OceanRemote allows per-relay logic settings
• Test each relay individually to determine correct logic

Technical Deep Dive

Why Negative Logic is Common in Cheap Modules

Many inexpensive relay modules use an optocoupler (PC817) and an NPN transistor. The optocoupler's LED is connected between VCC and the IN pin. When IN is LOW (0V), current flows through the LED, activating the optocoupler and turning on the relay. When IN is HIGH (3.3V/5V), no current flows (or very little), and the relay stays OFF.

Negative Logic Circuit (Simplified):
         
        VCC (5V)
          │
          ▼
        ┌───┐
        │LED│  ← Optocoupler LED
        └───┘
          │
          ├──────► IN pin from microcontroller
          │
         GND
         
When IN is LOW: Current flows through LED → Relay ON
When IN is HIGH: No current flows → Relay OFF

Positive Logic Modules

These modules typically have an additional transistor that inverts the signal, or they use a different optocoupler configuration where the LED is driven directly by the IN pin.

Positive Logic Circuit (Simplified):
         
        IN pin from microcontroller
          │
          ▼
        ┌───┐
        │LED│  ← Optocoupler LED
        └───┘
          │
         GND
         
When IN is HIGH: Current flows through LED → Relay ON
When IN is LOW: No current flows → Relay OFF

Troubleshooting Relay Logic Issues

Problem: Relay clicks when dashboard shows OFF

• Cause: Logic is reversed
• Fix: Change logic setting (Positive ↔ Negative)

Problem: Relay never clicks (always OFF)

• Check VCC and GND connections (relay needs 5V)
• Test with a multimeter to see if GPIO pin is changing state
• Try the opposite logic setting (maybe the relay needs the opposite state to click)
• Relay module may be damaged

Problem: Relay clicks but device doesn't turn on/off

• Check the high-voltage wiring (COM, NO, NC terminals)
• Verify you're using the correct terminals (NO = Normally Open, NC = Normally Closed)
• Check if the connected device is powered

Problem: Relay clicks erratically or buzzes

• Insufficient power supply (relay needs stable 5V)
• GPIO pin not providing enough current (use a transistor driver)
• Electrical interference (use shielded cables or add a capacitor)

Summary Table

Next Steps

Now that you understand relay logic, continue with:

• Tutorial 21: Connecting Relays to Your Board (wiring guide)
• Tutorial 23: Customizing Relay Names and Logic
• Tutorial 24: Using Multiple Relays: Best Practices
• Return to Tutorial 02: Generate firmware with correct logic settings!