This circuit was found in documentation from the 1970s. However, it can be implemented with modern transistors such as 2N2222 for Q1 and BF494 for Q2. The crystal depends on the desired frequency.
This circuit is from 1964 documentation but can be easily implemented based on current components....
This Power supply sources from 500 to 1,000 mA according the transformar and was found in a...
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....