This pin on the Arduino board provides the voltage reference with which the microcontroller operates. Maximum current draw is 1 A (according to the power input source).

A 3.3 volt supply generated by the on-board regulator. Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). This pin outputs a regulated 5V from the regulator on the board. You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). If using more than 12V, the voltage regulator may overheat and damage the board. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. The board can operate on an external supply of 6 to 20 volts.

Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. The power source is selected automatically.External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The Arduino UNO WiFi can be powered via the USB connection or with an external power supply.

(Or the device can act as an access point.) One useful feature of UNO WiFi is support for OTA (over-the-air) programming, either for transfer of Arduino sketches or Wi-Fi firmware. The ESP8266 Wi-Fi Module is a self contained SoC with integrated TCP/IP protocol stack that can give access to your Wi-Fi network. It contains everything needed to support the microcontroller simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB connection, a power jack, an ICSP header, and a reset button. The Arduino UNO WiFi is the same as a Arduino UNO Rev3 but with an integrated Wi-Fi module! The board is based on the ATmega328P with an ESP8266 Wi-Fi Module integrated (datasheet). This product is retired, check out the Arduino UNO WiFi Rev2

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