This simple signal generator with transistors was published in Radio Electronics magazine in November 1957. The circuit generates signals determined by the coil and modulation by the transformer.
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....