This circuit was found in a 1980s documentation, using a comparator from the time. The configuration can also be implemented with comparators or modern operational.
I found this circuit in a 1992 Electronics Experimenter Handbook. The circuit can be easily...
This egg timer has been found in an English magazine that no longer exists in 1982. The...
This comparator application circuit was obtained from the June 1986 Radio Electronics magazine. The...