This CMOS integrated alarm was found in the July 1992 Radio Electronics magazine. The circuit has a small power stage and its sensitivity is adjusted at R5.
This is the project found in a 1981 publication, but it is still viable. It is a simple AM...
This commercial earphone receiver was manufactured in the 1960s in Japan. It was powered by a 9 V...
This circuit was found in a European publication from the 1980s. The transistors can be the BC548...