The UVTRON tube is an ultraviolet radiation sensor. In this circuit from Radio Electronics from April 1990 we have a flame detector.
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...