Temperature Difference Indicator with LM35, BJT Differential Amplifier, and Arduino Nano

Monitoring temperature differences is crucial in various applications, from environmental control to electronic device safety. For example, compare indoor and outdoor temperatures to control heating/cooling systems, In this project, we’ll build a simple yet effective temperature difference indicator using LM35 temperature sensors, a BJT differential amplifier, and an Arduino Nano. This setup allows you to measure and visualize the temperature difference between two points using an LED indicator.

Components Needed

• LM35 Temperature Sensors (2x)
• NPN Transistors (e.g., 2N3904) for the differential amplifier
• Resistors (2.2k, 10k and 47k)
• Arduino Nano
• LED (for visual indication)
• Breadboard and Jumper Wires
• Power Supply (5V)

Understanding the LM35 and Differential Amplifier

The LM35 is a popular temperature sensor that outputs a voltage proportional to the temperature. Each sensor provides a 10 mV/°C output, making it easy to interface with analog circuits.

A differential amplifier is a circuit that amplifies the difference between two input signals. In this case, we’ll use a basic BJT differential amplifier to compare the outputs from two LM35 sensors. For a detailed explanation of how a differential amplifier works, check out our guide on the basic BJT differential amplifier.

Circuit Design

1. Setting Up the LM35 Sensors:

   • Connect the VCC pin of both LM35 sensors to the 5V supply.
   • Connect the GND pin to the ground.
   • Connect the output pin of the first LM35 sensor to the base of the first transistor in the differential amplifier.
   • Connect the output pin of the second LM35 sensor to the base of the second transistor.

2. Building the Differential Amplifier:

   • Use two NPN transistors (e.g., 2N3904) for the differential amplifier.
   • Connect the emitters of both transistors together and then to ground through a common emitter resistor.
   • Connect the collectors of each transistor to the power supply (5V) through collector resistors.
   • Take the differential output from the collector of one of the transistors, which will be fed into the Arduino Nano's analog input.

3. Arduino Nano and LED Indicator:

   • Connect the output of the differential amplifier to one of the analog input pins (e.g., A0) of the Arduino Nano.
   • Connect an LED to one of the digital pins (e.g., D13) with a current-limiting resistor.
    

const int ledPin = 13; // LED connected to digital pin 13
const int sensorPin = A0; // Differential amplifier output connected to analog pin A0

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int sensorValue = analogRead(sensorPin);
  int blinkRate = map(sensorValue, 0, 1023, 100, 1000); // Map the sensor value to a blink rate between 100ms and 1000ms
  
  digitalWrite(ledPin, HIGH);
  delay(blinkRate);
  digitalWrite(ledPin, LOW);
  delay(blinkRate);

  Serial.print("Sensor Value: ");
  Serial.println(sensorValue);
}