Suggested by an American documentation from the 90s, this circuit generates signals with frequencies determined by the capacitors and calculated by the formula next to the diagram.
According to the author of this project, from a documentation on espionage from the 1980s, this...
This circuit suggested by Siemens can provide time intervals for triggering values that are given...
This circuit generates a good-intensity 1 kHz rectangular signal. The circuit is from Electronics Now...