This circuit is from a 1969 EEE magazine. The transistors are from that time, but the circuit can be implemented with modern equivalents, such as 2N2646 for Q3.
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...