This harmonic oscillator controlled by crystal was found in an American documentation of the 90's. The circuit uses transistors of the time. Current equivalents can be used.
February 25, 2015 - Mouser Electronics, Inc. today announced their new Robotics Technology site....
This circuit was found in an Australian publication from 1991. The circuit emits a tone as well as...
This circuit is from an old publication but can be implemented with modern silicon transistors such as...