The pinout diagrams are simple, but they're easy to read, and especially helpful for beginners. You can print them out full size so you can set your Arduino Uno, ATMega, or ATTiny right on top for ...

The Arduino Nano and Uno are equipped with very similar processors (the chip that essentially serves as the brain of the board). The Nano features an ATmega328, while the Uno sports an ATmega328P.

Not exactly schematics, but [Duckman] has some Arduino pinout diagrams he made using ASCII-art. These could be useful when pasted into source code as comments, documenting the pinouts for your ...

While the pinout is very similar to the Arduino Nano’s, there are four (12-bit) analogue inputs rather than eight (10-bit). Some digital GPIO pins are numbered differently too.

An Arduino Nano or a Micro might be the best options If a small board sounds like the most appropriate choice.