Found in a 1969 British publication, this circuit uses a regenerative switch with two transistors to produce pulses that depend on C1. The relay must be in accordance with the supply voltage.
The circuit shown was found in the National Semiconductor's Linear Applications Handbook of 1994....
This circuit from a Popular Electronics shows how to take the audio signal from a telephone line...
This circuit is from a Radio Electronics from April 1972. The circuit uses power transistors of...