The 2N3904 is a common NPN bipolar junction transistor used for general purpose low-power amplifying or switching applications. The type was registered by Motorola Semiconductor in the mid-sixties, together with the complementary PNP type 2N3906, and represented a significant performance/cost improvement, with the plastic TO-92 case replacing metal cans. It is designed for low current and power, medium voltage, and can operate at moderately high speeds. This transistor is low cost, widely available and sufficiently robust to be of use by experimenters. When looking at the flat side with the leads pointed downward, the three wires emerging from the bottom are connected to, left to right, the emitter, the base and the collector. Some manufacturers mark "EBC" on the molded part, but all are required to have those connections for a part which is a "2N3904".
ATmega328 microcontroller with Optiboot (UNO) Bootloader
USB Programming and debugging via the well-supported genuine FTDI FT231X
Input voltage (Vin): 6-16V (a 9VDC power supply is recommended)
5V logic with 3.3V compatible inputs, can be converted to 3.3V logic operation
20 Digital I/O Pins: 6 are also PWM outputs and 6 are also Analog Inputs
32KB Flash Memory - 0.5K for bootloader, 31.5KB available after bootloading
16MHz Clock Speed
Adafruit Black PCB with gold plate on pads
18mm x 44mm x 4mm / 0.7" x 1.7" x 0.2"
Derivative of "Arduino UNO R3 Reference design"
Open source hardware files on github!
The Uno is a microcontroller board based on the ATmega328P. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz quartz crystal, a USB connection, a power jack, an ICSP header and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.. You can tinker with your UNO without worrying too much about doing something wrong, worst case scenario you can replace the board for a few dollars and start over again.
"Uno" means one in Italian and was chosen to mark the release of Arduino Software (IDE) 1.0. The Uno board and version 1.0 of Arduino Software (IDE) were the reference versions of Arduino, now evolved to newer releases. The Uno board is the first in a series of USB Arduino boards, and the reference model for the Arduino platform.