We found this circuit in a 1980s publication on telephony. The circuit uses an integrated amplifier from that time, which is uncommon today.
This circuit of a 1969 Electronic Design uses RTL logic. The setting can be adapted to TTL. The...
This circuit can be found in other projects in this site. When S1 is open the circuit produces...
This interface shield circuit is from the 1983 Intersil Databook. The components used are uncommon...