Here is a simple, yet useful circuit of an Arduino 4-digit 7-segment LED display unit ... display section is wired around four common-cathode 7-segment LED displays, and four BC547 npn transistors.

This is an easy tutorial showing how you can control the brightness of an LED or adjust the speed of a DC fan using Arduino Board, a small BC547 transistor and a potentiometer ... Do not power the ...

[Eric] uses the oscillator to build a simple LED blinker and shows what happens when he changes the transistor and adjusts the voltage or resistance. He also shows the circuit as a tone generator ...