NPK-S Sensor Arduino UNO Integration Tutorial

This tutorial shows you how to integrate the NPK-S (RS485) soil sensor with Arduino UNO to read Nitrogen, Phosphorus, and Potassium levels from soil.

What You'll Need

Hardware

• Arduino UNO
• NPK-S Sensor (RS485 type)
• RS485 to TTL converter module (MAX485 or similar)
• Breadboard and jumper wires
• 5V power supply (or use Arduino's 5V output)

Software

• Arduino IDE
• SoftwareSerial library (included in Arduino IDE)

Sensor Specifications

Wiring Diagram

NPK-S Sensor Cable Colors

• Brown: Power + (5V from Arduino or external supply)
• Black: Power - (GND)
• Yellow: RS485 A+
• Blue: RS485 B-

Connections

NPK-S Sensor          MAX485 Module          Arduino UNO
─────────────────────────────────────────────────────────
Brown (Power+)  ───>  VCC (5V)         ───>  5V
Black (Power-)  ───>  GND              ───>  GND
Yellow (A+)     ───>  A
Blue (B-)       ───>  B
                      RO               ───>  Pin 10 (RX)
                      DI               ───>  Pin 11 (TX)
                      DE & RE (tied)   ───>  Pin 8 (Control)
                      VCC              ───>  5V
                      GND              ───>  GND

Note: Connect DE and RE pins together on the MAX485 module and connect to Pin 8 for transmit/receive control.

Communication Protocol

The sensor uses Modbus RTU protocol with these parameters:

• Baud Rate: 4800
• Data Bits: 8
• Parity: None
• Stop Bits: 1
• Function Code: 0x03 (Read Holding Registers)

Register Addresses

• Nitrogen: 0x001E (30 decimal)
• Phosphorus: 0x001F (31 decimal)
• Potassium: 0x0020 (32 decimal)

Arduino Code

#include <SoftwareSerial.h>

// RS485 control pin
#define DE_RE_PIN 8

// Software Serial pins for RS485 communication
#define RX_PIN 10
#define TX_PIN 11

// Create software serial object
SoftwareSerial modbus(RX_PIN, TX_PIN);

// Sensor address
const byte sensorAddress = 0x01;

// CRC calculation for Modbus RTU
uint16_t calculateCRC(uint8_t *data, uint8_t length) {
  uint16_t crc = 0xFFFF;
  for (uint8_t i = 0; i < length; i++) {
    crc ^= data[i];
    for (uint8_t j = 0; j < 8; j++) {
      if (crc & 0x0001) {
        crc >>= 1;
        crc ^= 0xA001;
      } else {
        crc >>= 1;
      }
    }
  }
  return crc;
}

// Read NPK values from sensor
bool readNPK(uint16_t &nitrogen, uint16_t &phosphorus, uint16_t &potassium) {
  // Modbus request frame to read 3 registers starting from 0x001E
  uint8_t request[] = {
    0x01,       // Slave address
    0x03,       // Function code (Read Holding Registers)
    0x00, 0x1E, // Starting address (0x001E = Nitrogen)
    0x00, 0x03, // Number of registers to read (3)
    0x00, 0x00  // CRC (will be calculated)
  };
  
  // Calculate and add CRC
  uint16_t crc = calculateCRC(request, 6);
  request[6] = crc & 0xFF;        // CRC Low
  request[7] = (crc >> 8) & 0xFF; // CRC High
  
  // Set to transmit mode
  digitalWrite(DE_RE_PIN, HIGH);
  delay(10);
  
  // Send request
  modbus.write(request, 8);
  modbus.flush();
  
  // Set to receive mode
  digitalWrite(DE_RE_PIN, LOW);
  
  // Wait for response
  unsigned long startTime = millis();
  while (modbus.available() < 11 && millis() - startTime < 1000) {
    // Wait for response with 1 second timeout
  }
  
  // Check if we received enough bytes
  if (modbus.available() < 11) {
    Serial.println("Error: Timeout waiting for response");
    return false;
  }
  
  // Read response
  uint8_t response[11];
  for (int i = 0; i < 11; i++) {
    response[i] = modbus.read();
  }
  
  // Verify response
  if (response[0] != sensorAddress || response[1] != 0x03) {
    Serial.println("Error: Invalid response");
    return false;
  }
  
  // Verify CRC
  uint16_t receivedCRC = response[9] | (response[10] << 8);
  uint16_t calculatedCRC = calculateCRC(response, 9);
  
  if (receivedCRC != calculatedCRC) {
    Serial.println("Error: CRC mismatch");
    return false;
  }
  
  // Extract NPK values (response bytes 3-8 contain the data)
  nitrogen = (response[3] << 8) | response[4];
  phosphorus = (response[5] << 8) | response[6];
  potassium = (response[7] << 8) | response[8];
  
  return true;
}

void setup() {
  // Initialize serial communication with PC
  Serial.begin(9600);
  Serial.println("NPK-S Sensor Reader");
  Serial.println("==================");
  
  // Initialize RS485 control pin
  pinMode(DE_RE_PIN, OUTPUT);
  digitalWrite(DE_RE_PIN, LOW); // Start in receive mode
  
  // Initialize Modbus communication
  modbus.begin(4800);
  
  delay(1000);
}

void loop() {
  uint16_t nitrogen, phosphorus, potassium;
  
  Serial.println("\n--- Reading NPK Sensor ---");
  
  if (readNPK(nitrogen, phosphorus, potassium)) {
    Serial.print("Nitrogen (N):    ");
    Serial.print(nitrogen);
    Serial.println(" mg/kg");
    
    Serial.print("Phosphorus (P):  ");
    Serial.print(phosphorus);
    Serial.println(" mg/kg");
    
    Serial.print("Potassium (K):   ");
    Serial.print(potassium);
    Serial.println(" mg/kg");
  } else {
    Serial.println("Failed to read sensor data");
  }
  
  // Wait 5 seconds before next reading
  delay(5000);
}

How It Works

1. Initialization: The Arduino sets up serial communication for debugging (9600 baud) and Modbus communication with the sensor (4800 baud).

2. Request Frame: To read NPK values, the Arduino sends a Modbus RTU request:

   • Address: 0x01 (sensor address)
   • Function: 0x03 (read holding registers)
   • Start Register: 0x001E (Nitrogen)
   • Number of Registers: 3 (N, P, K)
   • CRC: Calculated checksum

3. Response Handling: The sensor responds with 11 bytes containing:

   • Address and function code (2 bytes)
   • Byte count (1 byte)
   • NPK data (6 bytes: 2 each for N, P, K)
   • CRC checksum (2 bytes)

4. Data Extraction: Values are extracted from the response and displayed in mg/kg.

Advanced Features

Changing Sensor Address

To change the sensor address from 1 to 2:

uint8_t setAddress[] = {0x01, 0x06, 0x07, 0xD0, 0x00, 0x02, 0x08, 0x86};

Changing Baud Rate

To set baud rate to 9600:

uint8_t setBaud[] = {0x01, 0x06, 0x07, 0xD1, 0x00, 0x02, 0x59, 0x46};

Expected Output

When running successfully, you'll see output like this in the Serial Monitor:

NPK-S Sensor Reader
==================

--- Reading NPK Sensor ---
Nitrogen (N):    32 mg/kg
Phosphorus (P):  37 mg/kg
Potassium (K):   48 mg/kg

--- Reading NPK Sensor ---
Nitrogen (N):    35 mg/kg
Phosphorus (P):  38 mg/kg
Potassium (K):   50 mg/kg