This is a pretty basic application of microcontrollers. Here, 2 Common Cathode 7 segment LEDs have been driven by an Atmega8 (Atmel's AVR Micro controller). The circuit has 2 npn transistors BC547 acting as current sinks for the LEDs. The circuit employs multiplexing to drive the two LEDs (saves the number of pins). The underlying concept is Persisence of Vision ie the eye cannot distinguish motion which has a frequency greater than 24 Hz. Hence each LED is run at a frequency which is significantly greater than the 24Hz. Hence it seems as if the LEDs are running simultaneously even though they are being lit individually in quick succession.
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