This CMOS circuit was found in a Popular Electronics from the 80's. The circuit consists of a counter that has its output activated when the selected key combination is pressed.
This version is one of many that can be found on our website. We found it in Radio Electronics from the...
This circuit for common analog telephones was found in March 1982 Radio Electronics magazine. It...
Figure bellow shows how the 4510, a divide-by-1U, up/down counter with three-state outputs is...