This unijunction transistor circuit is a frequency divider synchronized to 100 kHz. The circuit was found in a Radio Electronics from July 1968. The adjustment of R4 is important.
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....