This circuit is from a 1972 Motorola publication. The used transistor can still be found, but it can be used equivalent. The circuit is powered by the amplifier with which it must operate.
This circuit of a 1969 Electronic Design uses RTL logic. The setting can be adapted to TTL. The...
We found this circuit in a 1976 documentation, but it can still be easily assembled using BC548...
This interface or shield circuit for instrumentation was obtained in a publication from the 90's....