Microcontroller ADC

int raw = analogRead(A0);              // 0–1023 on a 10-bit Arduino Uno
float volts = raw * 5.0 / 1023.0;       // (or / 1024, see datasheet)

Tips for accurate readings:

• use a stable reference ($V_{ref}$), not a noisy supply;
• keep the source impedance low (≤ 10 kΩ for AVR) or add a buffer;
• average several samples to reduce noise;
• don't exceed $V_{ref}$ or the supply on the pin — divide higher voltages down.

ESP32 ADCs are notably non-linear; calibrate them or use an external ADC (ADS1115) for precision.