This small tone generator was found in the November 1987 issue of Radio Electronics magazine. The phototransistor is common and controls the sounds generated. The basic frequency is given by C1.
This circuit, which presents a certain danger and should be used with caution, was found in the...
This circuit with variable capacitance diode (varicap) is from a publication from the 90s. The...
This circuit, shown in the figure, consists of a variable gain amplifier with differential input...