Using LM3909 integrated circuits, this circuit was found in an American documentation from the 90's. The frequencies can be changed by changing the capacitors.
This 2000 circuit can work with one of the many infrared remote-control transmitters described in this...
This receiver for listening on the phone was found in a Japanese magazine in 1967. The circuit uses...
The circuit shown in the figure is suggested by Intersil (www.intersil.com) and makes use of the...