We found this circuit in an old American publication from the 1980s. Transistors can be general-purpose types like the BC548, and the tuning frequency is given by the L and C components.
This circuit consists of a low-pass filter for the frequency of 440 Hz. The filter therefore...
This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical...
This astable multivibrator with an operational one at the time was found in the National...