The difference between this circuit and the previous one is that it uses PNP transistors. The circuit is from Radio Electronics magazine from July 1968. The frequency depends on R1 and C1.
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....