LM35 and TMP36: an Analog Thermometer on ESP32 and Arduino Without Libraries

About the project

The LM35 and the TMP36 are the simplest temperature sensors there are: a chip in a TO-92 package, like a transistor, outputs a voltage proportional to temperature. No bus, no address, no library — just analogRead(). That makes them a good way to learn how a microcontroller ADC behaves and where its errors come from.

What you need

• ESP32 DevKit (or an Arduino Uno / Nano)
• An LM35DZ or TMP36 sensor
• A few wires; for a long cable, a 0.1 µF capacitor between OUT and GND

LM35 or TMP36

The main LM35 trap is its 4 V minimum supply. It is not specified for 3.3 V, so on an ESP32 it goes to the 5V pin: the output never exceeds 1.5 V anyway, which is safe for an ESP32 input. The second catch is that on a single supply the LM35 cannot output a negative voltage, so it never shows frost. For outdoors use a TMP36 or a DS18B20.

Wiring

Holding the sensor with the flat side towards you, the legs from left to right are +Vs, Vout, GND.

On an ESP32 use the ADC1 pins (GPIO32–39): ADC2 does not work while Wi-Fi is on.

Why millivolts rather than analogRead()

The ESP32 ADC is non-linear: near zero and near the top it loses tens of millivolts, and the gain differs from chip to chip. analogRead() * 3.3 / 4095 easily gives 2–3 °C of error. analogReadMilliVolts() applies the factory calibration stored in eFuse and returns millivolts — for an LM35 that is tenths of a degree straight away.

On an Arduino Uno the ADC is linear but coarse: 10 bits against 5 V is 4.9 mV per step, about 0.5 °C. You can do better by switching the reference to the internal 1.1 V (analogReference(INTERNAL)): the step becomes 0.1 °C and the maximum 110 °C.

Code

// LM35 / TMP36 on the ESP32: average the readings and convert millivolts to °C
const int SENSOR_PIN = 34;     // ADC1
const bool IS_TMP36 = false;   // true for a TMP36 (0 °C = 500 mV)

float readCelsius() {
  uint32_t sum = 0;
  for (int i = 0; i < 16; i++) {
    sum += analogReadMilliVolts(SENSOR_PIN);   // millivolts with the factory calibration
    delay(2);
  }
  float mv = sum / 16.0;
  return IS_TMP36 ? (mv - 500) / 10.0 : mv / 10.0;
}

void setup() {
  Serial.begin(115200);
}

void loop() {
  float t = readCelsius();
  Serial.printf("Temperature: %.1f °C\n", t);
  delay(1000);
}

Sixteen readings 2 ms apart average out ADC noise and Wi-Fi interference. In the simulator change the temperature in the sensor properties and switch the model to TMP36 — then remember to change IS_TMP36 too.

Common problems

• It reads 5–10 °C too high. The sensor heats itself or picks up heat from the board. Move it away from the regulator on a wire.
• Readings jump on a long cable. The LM35 dislikes cable capacitance: add a 2 kΩ resistor in series with the output, or a 0.1 µF capacitor to GND at the ADC input.
• A 3.3 V supply. From 3.3 V the LM35 reads wrong or gets stuck — the simulator warns you about it.
• 0.5 °C steps on an Arduino. That is the 10-bit ADC; use analogReference(INTERNAL) or a digital sensor.