This shield circuit is from a 1968s Motorola documentation. The relay is according to the supply voltage. R1 can be reduced to lower trigger voltages.
September 2016 - can be found at Mouser Electronics the new amplifiers with digital variable gain...
This receiver was found in an article in Electronics Now magazine in March 1994. The coils are...
This alarm circuit has a trigger delay system. The circuit was found in a Popular Electronics of the 90s....