This circuit was obtained in a Popular Electronics from the 90's. The gain depends on R4 and R3. Transistors are general purpose types.
This circuit was found in a Mullard transistor manual from 1960. Note the symmetrical power...
With the shadow over the sensor, it is possible to interrupt the sound of your amplifier using this...
This circuit puts the battery in the circuit when the voltage of a source is cut, preventing it...