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 ...

His test circuit uses six 2N2222 transistors to protect the Arduino from excessive current. You can see six red LEDs above which are inline with the base of teach transistor. This gives visual ...