We had found this circuit in a January 1961 Radio Electronics Magazine. Using it is possible to take measures from the gain of transistor with a common a multimeter.
This alarm with sensors in series and parallel was found in the 1995 Encyclopedia of Electronic...
This integrated amplifier was used in televisions from the 70s, with good sensitivity...
This source, found in a publication from the 1980s, uses an integrated circuit that is uncommon today....