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.
We found this filter configuration in an old documentation. It is one of many possible ones that...
This circuit is from a 1964 Electronic Design book that presents application ideas for...
This linear circuit was found in a 1965 Electronic Design. The transistors are from that time....