Arduino board

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We use Arduino Uno as an example to illustrate the main circuit components of an Arduino board. Figure below shows the name of components. Table 1 shows the explanation for each component. 

Arduino Uno board cited from Nasreen, Sanjida & Rokunuzzaman, Md & Biswas, Samarjith. (2017). Assessment of Surface Roughness Using LVDT: A Convenient and Inexpensive Way of Measuring Surface Irregularities.
Table 1: Arduino Uno board components function.
Components Function
USB Plug connect to computer with USB cable for loading code. Also, can be used to power Arduino
External Power Supply supply Arduino power from external power such as batter pack. It also is called as “Barrel Jack”. You can power your Arduino either through USB or Barrel Jack.
Serial In (RX) & Serial Out (TX) These two pins are used to setup a serial communication with other devices such as a second Arduino. You can learn more detail about serial communication here
Analog In Pins The area of pins under the ‘Analog In’ label (A0 through A5 on the Uno) are Analog In pins. These pins can read the signal from an analog sensor (like a temperature sensor) and convert it into a digital value.
Digital Pins Digital pins (0 through 13 on the Uno) can be used for both digital input (like telling if a button is pushed) and digital output (like powering an LED).
PWM Some of digital pins have the tilde (~) next to it. These pins (Pin 3, 5, 6, 9, 10, 11 on the Uno) have a special output function called Pulse-Width Modulation (PWM). You can learn more detail about PWM here
Ground Pins These pins are labeled as “GND” on the board. There are several GND pins on the Arduino. Any of them can be used to ground your circuit with Arduino.
Voltage In Pin This pin is labeled as “vin” on the board. You can use it and a GND pin to power Arduino. 
5V/3.3V Pins The 5V pin supplies 5 volts of power and the 3.3V pin supplies 3.3 volts of power. You can use them to supply power to simple low-power components. 
Analog Reference Pin This pin is labeled as “AREF” on the board. Most of the time you can leave this pin alone. It is sometimes
used to set an external reference voltage (between 0 and 5 Volts) as the upper limit for the analog input pins.
Reset Button/Pin You can use reset button to reinitialize Arduino board by pushing it. You can also use reset pin to reset Arduino board though other control circuits.