This circuit was suggested by Texas Instruments in a 1968 publication. The transistors are of the time, and modern equivalents can be used.
This circuit, found in an American publication from the 1970s, produces an audible tone in a...
This circuit was found in a 2006 publication. It can serve as the basis for a microcontrolled...
By adding a transistor to the output and some polarization features we can obtain a higher output...