This 555-based circuit was found in the August 1976 issue of Radio Electronics magazine. The tone frequencies are provided by the capacitors connected to the 555 IC.
This is the traditional configuration of an inverter amplifier with operational. The operational...
In the figure we show how to implement a voltage-controlled oscillator. The circuit is from Texas...
Found in a 1967 American publication, this circuit with feedback for two transistors has a gain of...