This circuit was found in a Popular Electronics from the 90s. The transistors are general purpose, and the gain depends on the ratio between R2 and R1.
This is the basic configuration for using the ULN2001 and MC1441 and others of the same series in...
This interesting effect circuit was found in a 1982 publication. The sequence of musical notes is...
This circuit, published in the 1980s, can be applied to operational amplifiers of almost any type....