LM35 with ESP32 and Arduino: ADC Errors and Sourcing

1. Project output and test scope

Our goal is to see the ambient temperature on the Serial Monitor and compare it with a multimeter and a reference thermometer. We have chosen the training window of the code from 15 to 90 degrees; This is a software choice, not a claim on the sensor's nominal accuracy or range. Only test at room temperature first; Do not use flame, uncontrolled heater or city electricity for heating.

If this is your first sensor project, from Course of sensors in 7 steps Start and then to Temperature and humidity hub come back

2. LM35DZ is not the same as all family members

Specification reference is TI datasheet: analog output with factor of 10 mV/deg, allowable supply 4 to 30 V, and nominal range of LM35D grade is 0 to 100 deg. A simple single-supply circuit is proposed for the positive region from about 2 degrees up; Subzero measurements cannot be generalized with this connection. Original part specifications cannot be guaranteed for unnamed or replica samples.

3. converting voltage to temperature; Keep the calculation auditable

After a valid voltage measurement, the conversion relationship is T = Vout_mV / 10. Calculation example: 250 mV becomes 25 degrees and 320 mV becomes 32 degrees. These are not our physical test data.

A small voltage error has a noticeable effect: a difference of 8 millivolts means a difference of 0.8 degrees in the calculation result. For this reason, only printing a few decimal places is not a sign of accuracy. Whenever the temperature number becomes unreasonable, check the voltage itself and the common ground before changing the conversion factor.

4. Necessary equipment

• Raw LM35DZ with specified manufacturer and grounding; Do not replace the finished module without checking the schematic.
• Classic ESP32 or Arduino Uno R3, safe USB cable and board.
• Regulated 5V power supply and 100 nanofarad capacitor for power bypass.
• Multimeter, short wire and reference thermometer with known accuracy.

The exact board number is important: ESP32-S3 and C3 have different ADC pins and behavior. The code intentionally gives a compile error for these models so unchecked wiring isn't accidentally used.

5. Secure connection to ESP32 and Arduino

For the LP/TO-92 package, match page 3 of the datasheet according to Bottom View: Pin 1 is supply, 2 is output, and 3 is ground. Do not confuse the view from below with the front view. Put the bypass capacitor near the power supply and ground of the sensor; This capacitor is not on the output.

If you have a separate 5V supply, share the grounds; Do not connect the output of two 5V sources together. Disconnect the power before moving the wires. Do not proceed if the part becomes abnormally hot.

6. Why is raw × 3.3 ÷ 4095 not enough?

In the Espressif official API, analogRead gives the raw data and analogReadMilliVolts the calibrated conversion to millivolts. For classic ESP32, the documented range of setting ADC_2_5db is about 100 to 1250mV; The code uses the same setting and 12-bit resolution. Do not extend this model table to S3 or C3.

The 150-900 mV window for this example is our conservative choice for practice. Internal ADC calibration is not a substitute for comparing the entire system with a reference. On the Uno, the converter is 10-bit and the default reference is power-dependent; Measure the actual value of 5V with a multimeter relative to GND and under normal load. Do not connect any external source to AREF in this program.

7. Temperature display code with averaging

One sample is produced every 20 milliseconds and after 16 samples an output is produced. Reading outside the window clears the buffer; The previous number is not printed as a new measurement. Averaging is for smoothing the fluctuation and does not correct for static error.

On the Uno, first change UNO_VREF_MV from zero to the supply voltage measured in millivolts; Until then the message CONFIG_REQUIRED is normal. Download the original code, RAW code and connection guide. Serial rate: 115200.

#include <Arduino.h>

// Educational display only. No heater, relay, or safety control.
#if defined(ARDUINO_ARCH_ESP32) && defined(CONFIG_IDF_TARGET_ESP32)
constexpr uint8_t SENSOR_PIN = 34;
#elif defined(ARDUINO_AVR_UNO)
constexpr uint8_t SENSOR_PIN = A0;
// Set to measured board supply in millivolts; 0 deliberately disables temperature.
constexpr uint16_t UNO_VREF_MV = 0;
#else
#error "Select classic ESP32 or Arduino Uno R3; other boards need reviewed ADC settings."
#endif

constexpr uint8_t SAMPLE_COUNT = 16;
uint32_t lastSample = 0;
uint32_t sumMv = 0;
uint8_t count = 0;

void setup() {
  Serial.begin(115200);
#if defined(ARDUINO_ARCH_ESP32)
  analogReadResolution(12);
  analogSetPinAttenuation(SENSOR_PIN, ADC_2_5db);
#else
  analogReference(DEFAULT); // Do not connect an external voltage to AREF.
#endif
  Serial.println(F("LM35 display: application window 15..90 C; not a safety monitor."));
}

void loop() {
  const uint32_t now = millis();
  if (now - lastSample < 20) return;
  lastSample = now;
  uint32_t mv;
#if defined(ARDUINO_ARCH_ESP32)
  mv = analogReadMilliVolts(SENSOR_PIN);
#else
  if (UNO_VREF_MV < 4500 || UNO_VREF_MV > 5500) {
    if (now % 1000 < 20) Serial.println(F("CONFIG_REQUIRED: measure VCC and set UNO_VREF_MV."));
    return;
  }
  mv = (uint32_t(analogRead(SENSOR_PIN)) * UNO_VREF_MV + 512) / 1024;
#endif
  // Plausibility window, NOT reliable open-wire or short-circuit detection.
  if (mv < 150 || mv > 900) {
    count = 0; sumMv = 0;
    Serial.print(F("OUTSIDE_DEMO_WINDOW mV=")); Serial.println(mv);
    return;
  }
  sumMv += mv;
  if (++count < SAMPLE_COUNT) return;
  const float averageMv = float(sumMv) / SAMPLE_COUNT;
  Serial.print(F("mV=")); Serial.print(averageMv, 1);
  Serial.print(F(" C=")); Serial.println(averageMv / 10.0f, 1);
  count = 0; sumMv = 0;
}

The RAW sample is for separating the wiring problem from the temperature conversion. In ESP32, raw and printed mV are two separate conversions and should not be assumed to be exactly the same sample. These two programs are compiled on Uno and ESP32; Not tested on physical hardware.

8. Start the test with the multimeter

1. Disconnect the power supply, match the package name and pins and check for unwanted short circuit.
2. turn on the sensor; First measure VOUT without connecting to GPIO. If it is near the power rail, do not continue to connect to the ESP32.
3. After power off, connect the output to ADC. Power on again, run the RAW code and compare the board voltage with a multimeter.
4. Run the main code; Place the reference thermometer and the LM35 side by side away from the board regulator and wait for stabilization.
5. Record multimeter voltage, program millivolts and reference temperature in several stable conditions; Do not generalize the result of one point to all temperatures.

The raw body is not waterproof; Do not submerge it in water. Touching the sensor also changes the test conditions; Do not report the touch reading as "room temperature".

9. Troubleshooting table before replacing the sensor

The filter in this example is not a definitive detection of sensor disconnection. For real control, fault detection, fault output behavior and independent protection must be designed and tested separately; This article covers measurement display only.

10. When should we go for another option?

For ADC learning and bench testing with short wires, the LM35 is a reasonable training option. If you don't want to handle the analog conversion in software, check out the DS18B20 tutorial. If you also need humidity, the SHT31 guide is a related topic. For another analog input exercise, see Light and Color Hub. The final choice must match the installation environment and the exact specifications of the sensor model.

11. shopping in Iran; Raw piece, inventory and originality

Review of links: 24 Shahrivar 1405, equal to 09/15/2026. The existence of a sales list in two stores and practical questions from buyers about temperature measurement and control is a sign of practical application in the Iranian market; Sales statistics or customer guarantee for every project. Get the specs from the datasheet, not the title or shop description.

In the ECA FAQ, an old seller's answer described the product as a "high copy". This answer does not prove the status of the current shipment, but it is reason enough not to assume authenticity. Before buying, ask the manufacturer and authenticity of the available series from the store; A traceable and documented part is required for guaranteed accuracy.

Inventory changes and we have not included a fixed price. LM35 is an analog sensor; The store title "digital" does not make it a digital sensor. The ready-made module or wired probe is not equivalent to the same raw piece without checking the circuit and insulation.

12. Common questions

13. Sources and continuation path

• Texas Instruments — LM35, Datasheet Rev. H: grid reference, bases, power supply and application circuit.
• Espressif — Arduino ESP32 ADC API: raw data, millivolts and attenuation.
• Arduino — analogRead: analog converter.
• Arduino — analogReference: reference selection and AREF considerations.

Next step: make a table of your actual measurements; Do not finalize the correction factor until the ADC difference, sensor error, and mounting effect have been separated. Other tutorials are available at Sensors Lab.