This circuit generates signals in four frequency bands with the threshold values indicated in the diagram. The circuit was found in the March 1996 issue of Electronics Now magazine.
The sensor in this circuit is a common transistor like the BC548. The circuit is from a 1979...
With this circuit it is possible to control a relay up to 100 mA with an extremely low input...
Found in an Electronic Design of 1964, this circuit is intended to the signal amplification in the 88 to 108...