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ESP8266 DISCUSSION
Different ESP8266 board tutorials quote different maximum voltages for A0. How can the actual input range be established without damaging the chip, and why is a development-board label insufficient evidence?
Reference: Espressif: ESP8266EX datasheet (https://documentation.espressif.com/0a-esp8266ex_datasheet_en.html).
Distinguish the ESP8266EX chip's TOUT input from the connector on a particular board. The chip-level external ADC range is 0 to 1 V, while some boards place a divider before it. Identify the exact board revision and inspect its schematic before applying an external voltage.
Include every divider stage when computing the input voltage. Two resistors added outside the board interact with any on-board resistance and source impedance. Use the complete circuit to calculate the ratio, then confirm it with a known low voltage that stays safely within the established range.
After checking the circuit, measure several reference points and account for ADC and resistor errors. Do not infer a board's safe limit from another board's example code or from a reading that happens to saturate near the expected value. Saturation is a measurement limit, not input protection.
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