9 Electronics: Controller Design and Review

Temperature-Activated Fan Circuit

The transistor examples above give the parts. This challenge asks you to assemble them into one reviewable design: sensing, switching, protection, and code behavior.

Your Challenge: Design a circuit that automatically turns on a 12V DC fan (300mA) when temperature exceeds 30°C, using an ESP32 and LM35 temperature sensor.

Bring the visual evidence in Figure 9.1 into smart fan controller design. Its purpose is to reveal the LM35 in this fan controller is a three-lead analog sensor: supply, ground, and a voltage output that changes by 10 mV per degree Celsius. Its transistor-like package makes pin identification and thermal placement part of the circuit design, not details the ESP32 code can repair later. Photo: Nevit Dilmen, CC BY-SA 3.0 before a design claim is accepted.

Inspect the three leads visible in Figure 9.1: they correspond to supply, ground, and the analogue output. The caption’s 10 mV per degree Celsius links that output to the ADC calculation, while pin identification and thermal placement warn that the transistor-like package is not self-explanatory. Confirm the flat-face orientation against the datasheet and place the body in representative airflow; neither a swapped pin nor heat from the fan driver can be corrected later by ESP32 code.

Work through the design as one signal-and-power chain. First translate the LM35 voltage into a temperature threshold, then choose a switch that survives the fan’s stall current, add an inductive-current path for switch-off, and only then write the GPIO logic. The component list below is the evidence set for those linked decisions, not a menu of interchangeable parts.

Given Components:

• ESP32 (3.3V I/O, 12-bit ADC, max GPIO current 12mA)
• LM35 temperature sensor (outputs 10mV/°C, e.g., 300mV @ 30°C)
• 12V DC fan (running current: 300mA, stall current: 800mA)
• Power supply: 12V, 1A
• Available transistors: 2N2222 NPN BJT, IRLZ44N N-channel MOSFET
• Resistors: assorted 1kΩ - 100kΩ
• Diodes: 1N4001, 1N5819 Schottky

Step 1: Sensor Interface

The LM35 outputs 300mV at 30°C. ESP32 ADC reads 0-3.3V (12-bit = 4096 levels).

Question: Can you connect LM35 directly to ESP32 ADC?

• Hint: LM35 max output is 1.5V @ 150°C. ESP32 ADC reads 0-3.3V.

Step 2: Transistor Selection

The fan draws 300mA running, 800mA stall.

Questions:

• 2N2222 is rated for 600mA max collector current. Is it suitable?
• If using 2N2222, what base resistor is needed for 300mA collector current (assume β=100)?
• If using IRLZ44N MOSFET, what is the power dissipation at 300mA (Rds(on) = 0.022Ω)?

Step 3: Protection Circuit

DC motors generate inductive spikes when switched off.

Questions:

• Which diode is better for flyback protection: 1N4001 or 1N5819 Schottky? Why?
• Where should the flyback diode be placed (anode/cathode orientation)?

Step 4: ESP32 Code Logic

Study the conversion lines in this Arduino sketch, then modify the program to add hysteresis. You can test your solution on real ESP32 hardware or in the Wokwi online simulator (search for “ESP32 + LM35” templates).

// Fan controller challenge: verify the conversion and add hysteresis
const int tempPin = 34;  // ADC pin
const int fanPin = 25;   // GPIO output
const float adcReferenceV = 3.3;
const float adcMaxCount = 4095.0;
const float lm35VoltsPerC = 0.010;

void setup() {
  pinMode(fanPin, OUTPUT);
  digitalWrite(fanPin, LOW);
}

void loop() {
  int adcValue = analogRead(tempPin);

  // Convert ADC reading to temperature
  // LM35 outputs 10mV/°C, ESP32 ADC is 12-bit (0-4095) for 0-3.3V
  float voltage = (adcValue / adcMaxCount) * adcReferenceV;
  float tempC = voltage / lm35VoltsPerC;

  if (tempC > 30.0) {
    digitalWrite(fanPin, HIGH);  // Turn fan ON
  } else {
    digitalWrite(fanPin, LOW);   // Turn fan OFF
  }

  delay(5000);  // Check every 5 seconds
}

Your Tasks:

1. Draw the complete circuit schematic
2. Select BJT or MOSFET and justify your choice
3. Calculate all resistor values
4. Explain the ADC-to-temperature conversion in the starter code
5. Add hysteresis to prevent fan chattering (turn ON at 30°C, turn OFF at 28°C)

const int tempPin = 34, fanPin = 25;
const float onTemp = 30.0, offTemp = 28.0; // 2 °C hysteresis band

void setup() {
  pinMode(fanPin, OUTPUT);
  digitalWrite(fanPin, LOW);
  Serial.begin(115200);
}

void loop() {
  int adc = analogRead(tempPin);
  float voltage = adc * (3.3 / 4095.0);
  float tempC   = voltage / 0.01;          // LM35: 10 mV/°C

  static bool fan = false;
  if (tempC > onTemp && !fan)  { digitalWrite(fanPin, HIGH); fan = true;  }
  if (tempC < offTemp && fan)  { digitalWrite(fanPin, LOW);  fan = false; }

  Serial.printf("ADC %d | %.3fV | %.1f°C | Fan %s\n",
                adc, voltage, tempC, fan ? "ON" : "OFF");
  delay(5000);
}