This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit is based on a 741 that activates a buzzer with oscillator. R5 adjusts the sensitivity.
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...