This alarm circuit with NC sensors in series was taken from an article in Radio Electronics magazine from May 1971. The SCR can be any of the 106 series.
The oscillator in figure 1 can be used in alarm, call or warning systems, producing a sound of...
This circuit is from a 1973 documentation. In it we have the way of using a FET as an input of...
This circuit is for traditional old-style telephone lines found in an American 1980s...