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Arduino UNO R3 (A000066, original): specs and pinout

Original Arduino UNO R3 with an ATmega328P and an ATmega16U2 USB-serial chip, 14 digital pins, 6 analog inputs, USB-B and a power jack.

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Specs

Power

Pinout

Rows come from the official UNO R3 full pinout sheet (A000066, last update 6 Oct 2022). The power header, analog header and digital header are listed in the order drawn on the sheet; the digital header runs D0 to D13 as printed (the datasheet's JDIGITAL table lists D0 to D13, GND, AREF, SDA and SCL). This is the original Arduino board, so the official documents apply.

Build with it

Project ideas for the Arduino UNO R3 (A000066, original). Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.

• Temperature and humidity display: Six fresh AA cells give about 9 V into the power jack, inside the 7-12 V nominal range. The DHT22 runs from 3.3 V to 6 V, so it works from the board's 5 V. The LCD module is a 5 V I2C backpack on A4 and A5; check its supply on the silkscreen. Learning project.
• Soil moisture readout: The sensor's analog output goes to one of A0 to A5. The module is not documented, so read its supply voltage from the silkscreen before wiring. Power from a 6 x AA holder on the power jack. Learning project.
• Ultrasonic distance meter: The HC-SR04 is a 5 V part (15 mA working current) with a TTL echo, which matches the Uno's 5 V I/O. Trigger and echo use two digital pins; the LCD uses A4 and A5.
• Knob position display: The encoder's two channels and push button use three digital pins with the internal pull-ups. The module's supply is not documented, so use 5 V only after checking its silkscreen.

Common mistakes

• Feeding the VIN pad or power jack outside 6 V to 20 V (7-12 V is the nominal range in the Tech Specs) (source: Arduino datasheet A000066 p.5; Arduino official documentation (Tech Specs, read from the page data))
• Drawing more than 50 mA from the 3.3 V pin or more than 20 mA from an I/O pin (source: Arduino datasheet A000066 page 1)
• Connecting 3.3 V-only boards or sensors straight to the 5 V I/O pins: the board's I/O voltage is 5 V, so use a level shifter or a 3.3 V part (source: Arduino official documentation (Tech Specs, read from the page data))

Not covered by the official source

• The datasheet text says the ATmega328P runs 'up to 20 MHz'; the board's clock is 16 MHz per the Tech Specs, which is the value shown.
• The datasheet's JDIGITAL table labels the last two pins A4/SD4 and A5/SD5; the pinout sheet's D18/SDA and D19/SCL are used.
• Maximum power consumption is printed as 'xx' in the datasheet.

Sources

• Arduino datasheet A000066 (UNO R3 User Manual, A000066, revision 4, modified 07/11/2026)
• Arduino datasheet A000066 (UNO R3 full pinout, last update 6 Oct 2022)
• Arduino official documentation (Arduino UNO Rev3 hardware page, Tech Specs)