This circuit was found in an article in Electronics Now magazine from March 1997. The transistors are general purpose, and the indicator is a 1 mA full scale analog type.
In the figure we have the adapter circuit for connecting a headphone to the output of a stereo...
Found in the June 1972 issue of Radio Electronics magazine, this circuit generates signals that...
The output of this circuit must be connected to the input of an audio amplifier. The microphone is a small...