This circuit was obtained in an Electronics World of 1968. The operational amplifiers ones are types of the time, no longer used today. The circuit responds from less than 20 Hz.
This circuit uses unijunction transistors and drives a small speaker through an output...
This circuit, found in a publication from the 1980s, can have its output voltage determined by the...
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....