This circuit mimics the heartbeat with the softest sound played through a small speaker. The circuit is from an Electronics Now issue of January 1997.
This two-channel wired intercom was found in the April 1990 issue of Radio Electronics magazine....
This sensor triggers an oscillator emitting sound when a magnet is approached by a reed switch....
This circuit is based on the voltage drop of the diodes in conduction, in the order of 1.6 to 1.7...