This circuit was obtained in a Popular Electronics from the 90's. The gain depends on R4 and R3. Transistors are general purpose types.
Found in a 1990s US publication, this alarm circuit detects when a light beam is interrupted. Optical...
This circuit for a commercial compact digital multimeter was found in the March 1979 issue of...
The flasher we present was found in a 1982 publication, shown in the figure. What we have is a...