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ADC · ESP32

Fix: ESP32 analogRead not working with Wi-Fi on (ADC2)

analogRead() on GPIO 0, 2, 4, 12–15 or 25–27 returns 0 or garbage once Wi-Fi starts. Those are ADC2 pins; move the sensor to ADC1 (GPIO 32–39).

ESP32Updated 31 Oct 2026

Community draft — report a mistake

What you see

OutputOutput
Soil raw: 2310   (before WiFi.begin)
Connecting to Wi-Fi... connected
Soil raw: 0
Soil raw: 0
Soil raw: 0

This is a hardware limitation of the original ESP32, not a bug in your code: the ADC2 converter is shared with the Wi-Fi radio. The fix is simply to use an ADC1 pin.

Quick fix — try in order

  1. 1

    Move the sensor to an ADC1 pin

    On the ESP32 DevKit V1 use GPIO 32, 33, 34, 35, 36 (VP) or 39 (VN). They keep working with Wi-Fi on.

  2. 2

    Change the pin in your sketch

    Update the #define or const for the analog pin and re-upload.

  3. 3

    Remember 34–39 are input-only

    They are ideal for analog sensors but cannot drive outputs.

Why it happens

Why does Wi-Fi block ADC2?

On the original ESP32 the Wi-Fi driver uses the ADC2 hardware. While Wi-Fi is active, ADC2 reads fail and Arduino returns 0 or an error value.

Which pins are ADC2?

GPIO 0, 2, 4, 12, 13, 14, 15, 25, 26, 27. Filter the interactive pinout by ADC to see them.

Does this affect ESP32-S3 / C3?

Newer chips share the ADC differently, but ADC1 is still the safe choice for anything read while Wi-Fi runs.

Board-specific notes

ESP32 DevKit V1: Our pin checker warns about ADC2 pins when you tick “Wi-Fi used”.

Parts: Capacitive soil sensor · LDR module — pinout, voltage and library notes.

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