Can a simple circuit translate a number you press on a keypad into binary coded decimal (BCD) output? You build a circuit using CMOS logic gates that reads input from up to 16 switches. Each key press produces the matching BCD value on four output lines. A memory module can be added so the output stays visible after you release the key. LEDs show each BCD digit, though they are optional. The circuit runs on 5 to 15 volts DC. You assemble everything on stripboard using basic tools. A drill bit can cut the copper tracks by hand if you do not have a dedicated track cutter.