30. Interface LM35 temperature sensor

Key Peripherals Used:

• ADC1 Channel 0 (PA0): Reads the analog voltage from the sensor Vout pin.
• ADC2 Channel 1 (PA1): Reads the analog voltage of Vref connected to the GND pin of the sensor.
• USART2: For serial communication with a serial terminal to display the measured temperature.

STM32 Hardware Connection

• LM35 Sensor connection:
  • Connect VCC 5V to the sensor’s +Vs pin.
  • Connect the sensor’s Vout pin to PA0 (ADC1_IN0) for analog input.
  • Create a voltage divider using R1 and R2 and apply 5V and GND to it.
  • Connect the divided voltage to the sensor’s GND pin and PA1(ADC2_IN1).
• Use USART2 on the STM32 board for serial communication to display the measured voltage value on a serial terminal. This typically connects to the USB interface from the board to the PC.
• Power the STM32 board through the USB cable connected to your computer for both power and UART communication.

Circuit Diagram

STM32 Firmware Implementation

Project Setup in STM32CubeIDE

1. Create a Project
   • Open STM32CubeIDE, start a new project, and select the NUCLEO-F103RB board.
2. Basic Configuration (via CubeMX inside CubeIDE)
   • Clock: Use the default HSI oscillator with PLL enabled (as configured in SystemClock_Config).
   • GPIO: Enable clocks for PORTA, PORTB, PORTC, and PORTD.
   • ADC Configuration
     • Two ADC peripherals used: ADC1 and ADC2.
     • ADC1
       1. Channel: ADC_CHANNEL_0 (PA0).
       2. Mode: Single conversion, software-triggered.
       3. Resolution: 12-bit (0–4095).
     • ADC2
       1. Channel: ADC_CHANNEL_1 (PA1).
       2. Same settings as ADC1.
     • Sampling time: 239.5 cycles for better accuracy.
     • Both ADCs use polling mode for conversion.
   • USART2: Enabled at 115200 baud, 8-N-1.
3. Code Generation
   • CubeMX will automatically generate all the startup code, including:
     • HAL_Init() → Initializes HAL and system tick.
     • SystemClock_Config() → Configures system clock (HSI + PLL).
     • MX_GPIO_Init() → Initializes GPIO ports.
     • MX_USART2_UART_Init() → Configures UART2.
     • MX_ADC1_Init() → Configures ADC1 for analog input.
     • MX_ADC2_Init() → Configures ADC2 for analog input.
   • This code sets up the hardware and prepares the project for firmware development, so we only need to add our application logic in the user code sections

Code Snippets from main.c

ADC Initialization (MX_ADC1_Init and MX_ADC2_Init):

static void MX_ADC1_Init(void)
{
  ADC_ChannelConfTypeDef sConfig = {0};

  /** Common config
  */
  hadc1.Instance = ADC1;
  hadc1.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc1.Init.ContinuousConvMode = DISABLE;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc1.Init.NbrOfConversion = 1;
  if (HAL_ADC_Init(&hadc1) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_0;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_239CYCLES_5;
  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }

}

Similarly, for ADC2 also 

This configures ADC1 and ADC2 for single-channel operation with right-aligned 12-bit results.

UART2 Initialization (MX_USART2_UART_Init):

static void MX_USART2_UART_Init(void)
{
  huart2.Instance = USART2;
  huart2.Init.BaudRate = 115200;
  huart2.Init.WordLength = UART_WORDLENGTH_8B;
  huart2.Init.StopBits = UART_STOPBITS_1;
  huart2.Init.Parity = UART_PARITY_NONE;
  huart2.Init.Mode = UART_MODE_TX_RX;
  huart2.Init.HwFlowCtl = UART_HWCONTROL_NONE;
  huart2.Init.OverSampling = UART_OVERSAMPLING_16;
  if (HAL_UART_Init(&huart2) != HAL_OK)
  {
    Error_Handler();
  }
}

This configures USART2 for communication at a 115200 baud rate with 8 data bits, no parity, and 1 stop bit, supporting both transmit and receive.

Header includes:

#include<string.h>
#include<stdio.h>

Private defines and variables:

#define ADC_SAMPLES 100       // Number of samples for averaging
#define ADC_RESOLUTION 4096   // 12-bit ADC resolution
#define REFERENCE_VOLTAGE 3.28f //Measured Reference voltage

uint32_t adcValues[2];    // Buffer for two channels
char uartBuffer[64];   // Buffer for UART messages

Main Loop - Reading and Transmitting temperature

while (1) {
  uint32_t sumCh0 = 0, sumCh1 = 0;
  uint8_t validSamplesCh0 = 0, validSamplesCh1 = 0;
  float voltageDiff = 0;
  int16_t tempValue = 0;
  int16_t tempDecimalValue = 0;

  // Take multiple samples for averaging
  for (int i = 0; i < ADC_SAMPLES; i++) {
    // Read channel 0
    HAL_ADC_Start(&hadc1);
    if (HAL_ADC_PollForConversion(&hadc1, 20) == HAL_OK) {
      sumCh0 += HAL_ADC_GetValue(&hadc1);
      validSamplesCh0++;
    }

    // Read channel 1
    HAL_ADC_Start(&hadc2);
    if (HAL_ADC_PollForConversion(&hadc2, 20) == HAL_OK) {
      sumCh1 += HAL_ADC_GetValue(&hadc2);
      validSamplesCh1++;
    }
  }

  // Calculate average values only if we have valid samples
  if (validSamplesCh0 > 0 && validSamplesCh1 > 0) {
    adcValues[0] = sumCh0 / validSamplesCh0;
    adcValues[1] = sumCh1 / validSamplesCh1;

    // Calculate absolute difference once
    uint32_t adcDiff =
      (adcValues[0] >= adcValues[1]) ? (adcValues[0] - adcValues[1]) : (adcValues[1] - adcValues[0]);

    if (adcDiff != 0) {
      voltageDiff = (REFERENCE_VOLTAGE * adcDiff) / ADC_RESOLUTION;
    }

    if (voltageDiff != 0) {
      tempValue = voltageDiff * 100;
      tempDecimalValue = (int16_t)(voltageDiff * 10000) % 100;
    }


    // Format message based on which channel is higher
    if (adcValues[0] >= adcValues[1]) {
      sprintf(uartBuffer, "Temperature: %u.%u Degree Celsius \r\n",
              tempValue, tempDecimalValue);
    } else {
      sprintf(uartBuffer, "Temperature: -%u.%u Degree Celsius \r\n",
              tempValue, tempDecimalValue);
    }

    // Send Temperature value on UART
    HAL_UART_Transmit(&huart2, (uint8_t*)uartBuffer,
                      strlen(uartBuffer), HAL_MAX_DELAY);
  } else {
    // Handle error case if needed
    sprintf(uartBuffer, "ADC read error\r\n");
    HAL_UART_Transmit(&huart2, (uint8_t*)uartBuffer,
                      strlen(uartBuffer), HAL_MAX_DELAY);
  }

  HAL_Delay(1000);
}

Main Loop Workflow

• Loop Forever (while (1))
  • Continuously reads, processes, and sends temperature data.
• Read & Average ADC Samples
  • Takes multiple samples from Channel 0 (ADC1) and Channel 1 (ADC2).
  • Computes the average ADC value for each channel.
• Calculate Voltage Difference
  • Finds the absolute difference between the two ADC values.
  • Converts this difference to a voltage (voltageDiff).
• Compute Temperature
  • Extracts:
    • Integer part (tempValue = voltageDiff * 100).
    • Decimal part (tempDecimalValue = (voltageDiff * 10000) % 100).
• Determine Sign & Format Output
  • If Channel 0 ≥ Channel 1 → Positive temperature (e.g., "25.5°C").
  • If Channel 0 < Channel 1 → Negative temperature (e.g., "-10.2°C").
• Send Temperature via UART
  • Transmits a formatted string (or an error if ADC read fails).
• Wait 1 Second (HAL_Delay(1000)) before repeating.

Download Project

The complete STM32CubeIDE project (including .ioc configuration, main.c, and HAL files) is available here:

📥 Download Project

Hardware Connections 

• The voltage at the GND pin is 1.241 Volts, which will shift the negative voltage on the Vout pin to positive voltages by 1.241 Volts.

Arduino UNO Firmware Implementation

Let’s write code

• Read voltage on both Analog pins.
• Subtract the offset voltage (LM35 reference) 1.241 Volts.
• Convert voltage to temperature
• Print the temperature on the Serial Monitor. We can also connect an LCD & display temperature on it. For this task, we will use the Serial monitor.

Code

const int lm35_pin = A0;   /* LM35 O/P pin */
const int offset_pin = A1; /* LM35 GND pin */
float offset_voltage = 0;  /* offset voltage stored in milli-volts */

int temp_adc_val;
float temp_val;

void setup() {
  Serial.begin(9600);
}

void loop() {
  temp_adc_val = analogRead(lm35_pin);
  offset_voltage = analogRead(offset_pin) * (5000 / 1024); /* read offset voltage on A1 pin and convert it to milli-volts */

  temp_val = temp_adc_val * (5000 / 1024); /* Convert adc value to equivalent voltage (milli-volts) */
  temp_val = temp_val - offset_voltage;    /* remove the offset voltage */
  temp_val = (temp_val / 10);              /* LM35 gives output of 10mv/°C */

  Serial.print("Temperature = "); /* Print the temprature*/
  Serial.print(temp_val);
  Serial.println(" Degree Celsius");
  delay(1000);
}

Code explanation

• float temp_val = adcValue * (5000 / 1024) : converts ADC_value to voltage (milli volts).
• temp_val = temp_val - offset_voltage: removes the offset voltage, that is given to the GND pin.
• temp_val = (temp_val/10) : converts into degree Celsius (1 degree Celsius = 10mV).
   

Output

Hardware Setup

Output video

As we can see in the video, the temperature change is successfully detected and printed on the Serial monitor.