The integrated circuits used in this configuration are not common today. The circuit is from the 1983 Intersil Databook. The indicator is 8 digits.
This circuit for common analog telephones was found in March 1982 Radio Electronics magazine. It...
We found this circuit in an English publication from the 1980s. The transistor can be BC548 or...
Found in a 1967 RCA documentation, this circuit can be made with equivalent SCRs according to the...