This circuit can be used as a lockable shield. A pulse activates the relay that remains on even after the pulse disappears. The transistor is general purpose.
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...