This circuit is from 1999 documentation. However, it can still be easily assembled, as all components used are common. The sensors can have their quantity increased, being connected in parallel.
This circuit can drive a loudspeaker with near 1 W of Power from 6 to 9 V supplies. The circuit...
This “one channel” demonstration circuit was found in the April 1979 issue of Radio Electronics magazine....
This circuit is from an August 1984 Radio Electronics magazine. It monitors the state of a RAM...