The circuit uses an AT89C51 microcontroller to control LEDs connected to its ports. The AT89C51 is an 8-bit microcontroller with 4KB of flash memory and 128 bytes of RAM. LEDs are connected to port P0 and turned on/off by the microcontroller setting the port pin high or low. Resistors are used to limit current flowing to the LEDs from the microcontroller's 5V output. Seven segment displays can also be interfaced by connecting segments to a port and driving each digit's pattern to display numbers.