This circuit is from a 1965 General Electric manual. It uses components from the time that admit equivalents. The SCR can be the TIC106 and the LDR of any type.
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...