This alarm goes off if someone leaves a door open. We found it in the Encyclopedia of Electronic Circuits Volume 5 from 1995. The circuit uses components that are still common today.
Found elsewhere in this section, this circuit is from the April 1971 Radio Electronics magazine. Note...
With this circuit, a relay can be activated from an alternating signal with frequency up to a few...
The sound of this experimental musical instrument is produced by the movement of the hand in front...