This circuit was found in the March 1997 Electronics Now magazine. It uses a common analog indicator and the transistor is a general purpose one.
This stethoscope or hearing aid amplifier has excellent gain. It was found in Popular Electronics from...
Found in Fairchild documentation from the 90s, this circuit generates signals with the...
This configuration, widely used in our articles and on this website, uses a 555 to control a relay...