555 is sending constant pulse stream to the counter at the count up pin. The 555 pulse rate can be changed by changing the value of the 1uf cap. The AND GATE is connected to b and d of the 7segment decoder input (b = 2) + (d = 8) = 10, this resets the counter when a value of ten is reached. The same is true for the tens group, but the diode alows the tens group to reset both the tens and ones groups. The logic gates can be used to give an output for any value useing bianery decoding 1, 2, 4, 8. Added up down switch
