In this circuit there is a counter , a decoder and 4 driver bjt.
To drive an alphanumeric display(i.e. 7x5) or 4 led, it can use this scheme whit a frequency clock (counter's input) of more than 100Hz, eye's band. In this way, the uman eye don't see when the led is off, and we can spend less in bjt. At normal rate of time, every leds appear on.
Excuse me for my english.
