This circuit is from a Radio Electronics from December 1964. The components are from the time and modern equivalents for transistors and diodes can be used.
This circuit uses a crystal phone and can be built with more modern transistors like the BC548. The...
This circuit was obtained from an American publication of the 80's. The integrated circuit is of the...
We found this circuit in a 1995 documentation. It includes several important features for a test...