Summary

You built three CMOS 4000 chips from transistors after the TI datasheets and a visitor counter on them.

• CD4011 — four NANDs: both a NOT and a latch are made of them.
• The F/F flip-flop — load at once, flip on the clock, hold.
• CD4029 — a counter up and down, binary and decade, with preset and carry.
• CD4511 — a binary code into the seven segments of a digit.
• Direction — two beams and a latch: which is blocked first.
• The device — "inside" and "total in" displays on the same beams.

The device is 2,664 transistors.

What next

• Three digits. One more CD4029 and CD4511 for each display: CO of the tens → CI of the hundreds.
• Do not go below zero. Now 00 − 1 = 99. Think of how to forbid counting down when "inside" is at 00.
• An "out" display. A third display only for the way out: the same beams, CI of the units — IN.