Found in Popular Electronics, this sophisticated short-wave receiver requires a lot of skill from an eventual assembler. The coils are commercial types of the time, used in commercial radios.
The sensors in this alarm are reed-switches. In the diagram we have only one, but several can be...
We found this circuit in a Poptronics from 1998. The circuit can be extended to more competitors...
The circuit shown in the figure uses two of the four operational amplifiers found on the LM324...