This two-digit circuit was found in the 1999 Radio Experimenter Handbook. It uses the CA3161 and CA3162 A/D converter pair common at the time.
Two transistors on a regenerative switch are used to trigger a triac. This circuit is from the RCA...
This circuit uses a 555 (according to the diagram) to generate 50 ms tone bursts at a frequency of 1.5...
The radio presented was found in a 1980 documentation. It consists of 3 stages: the tuning circuit...