This circuit from a Popular Electronics from the 90's can generate signals from 0.03 Hz to 3 kHz depending on the value of the capacitor and resistor.
April 2017 - Texas Instruments bq51222 integrated circuit for chargers and other Qi wireless power...
Using two 567 PLLs, this circuit recognizes two audio tones in sequence providing a signal for a...
This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose...