This circuit uses a crystal phone and can be built with more modern transistors like the BC548. The original circuit is from a 1979 publication and is powered with only two AA or AAA batteries.
This configuration can be found in other articles on this site. This was obtained in 1998...
In the figure we show how to polarize a phototransistor to obtain sensitivity. The circuit is from...
This compact power supply can be used on the workbench for up to 1 A load. The transformer has 1 A secondary...