This digital circuit is from a Radio Electronics magazine from December 1991. It does not use a microcontroller, but rather discrete digital circuits of the time.
This circuit is from a 1974 publication, but it can still be easily assembled today with BC548...
Found on Popular Electronics in the 1980s, this circuit detects a change in air pressure using a...
This 3-transistor amplifier stage was found in documentation from 1963. The transistors are made...