This circuit is from old documentation, using a component that cannot be found easily today. The indicator is a galvanometer, and the input voltage is in the order of 1 V.
This circuit was found in a 1977 magazine, but it can be easily assembled today using BC548...
This flip-flop or bistable was found in a publication about transistors from the 60's. The circuit...
This low-pass filter with 741 op and symmetrical source was found in the August 1993 issue of...