This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit equivalent and the relay is of the sensitive type for low voltage, according to the power supply.
This circuit for conventional lines was obtained in a 1996 documentation. The circuit uses common...
This tube circuit was found in the magazine 73 for radio amateurs in an issue from the 1980s. Note...
September 2016 - can already be found at Mouser Electronics new integrated circuit from Texas...