This old 4-channel circuit was found in the December 1994 Electronics Now magazine. The circuit uses a 556 as a modulator. Note the code programming key.
This circuit was suggested by a 1979 documentation, and a ceramic transducer can be used in place...
This is the previous version, but with a simplification given by the use of the 7404 and 7408...
Found in Popular Electronics from the 90s, this generator uses integrated circuits from that time....