This circuit was found in an American documentation from 1969. It serves to trigger circuits by providing a 7.5 V signal at the output from 500 mV pulses.
Using a 555, this configuration appears elsewhere on this site. This is from American...
This circuit monitors an old-style analog telephone line. We found it in the September 1991 issue...
Using the well-known SAA1027, this circuit serves as interface for PC. No English documentation was found...