Espressif ESP8266 User Manual

F. Measuring analog signals
So far we can read and write the digital states HIGHand LOW, but what if we want to deal
with analog signals? The ESP has one Analog To Digital Converter (ADC) which can be used
to measure voltage in the range 0 - 1V. To do that use the following command:
unsigned value = analogRead(A0);
You can also use the ADC to measure the input voltage without any additional wiring. You
have to instruct the processor that you want to measure the supply voltage rather than the
value on A0 with a special command outside the setup() and loop() method. Here is an
example:
ADC_MODE(ADC_VCC);
void setup()
{ Serial.begin(115200);
}
void loop()
{ Serial.println(ESP.getVcc());
delay(500);
}

Espressif ESP8266 Specifications

General

Digital I/O Pins	17
Analog Input Pins	1
Wi-Fi	802.11 b/g/n
Operating Voltage	3.0V - 3.6V
Clock Speed	80 MHz / 160 MHz
Operating Temperature	-40°C to 125°C
Dimensions	24mm x 14mm
Processor	Tensilica L106 32-bit micro controller