This circuit generates the busy tone of ordinary telephones. The transistors can be the BC548 and the components of the T filter and the multivibrator can be changed to obtain the exact tone.
Found in a 1968 EEE, this circuit accepts frequencies from 1 to 1000 Hz and control voltages from 0.1 to...
This circuit has appeared several times on this site in different applications. This is from 1997...
This setting appears other times in this section from other sources. This is from an 80's American...