This circuit was obtained in an 80s documentation on telephony. The components used are TTL, and the power must be made with 5 V.
This circuit was found in a Radio Electronics magazine from November 1992. The components are from...
This circuit was found in a 1969 Mullard publication. The transistor may be the modern equivalent...
This simple expandable circuit was obtained from a Popular Electronics from the 80's. The supply...