The A on the Arduino development board is the analog input pin. Arduino can read the analog signal by reading the level value on the input pin.It can accept 0 ~ 5V analog signal on Arduino UNO.The analog input pin is a pin with ADC (Analog-Digital Converter) function, which can convert the externally input analog signal into a digital signal that can be recognized during chip operation, thereby realizing the function of reading in analog values .The analog input function of Arduino has 10-bit precision, which can convert the voltage value of 0 ~ 5V into an integer form of 0 ~ 1023.The analog signal input function needs to use the analogRead () function.int value = analogRead (pin);pin refers to the pin of analog input, the specified pin must be the pin of analog input such asint value = analogRead (A0); // Read the analog signal on the A0 pin