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.
The circuit shown in the figure can serve as a basis for frequency meters, clocks. Stopwatches and many other...
This circuit, obtained from a 1965 magazine, can still be assembled using some modern equivalent...
This variable gain circuit for analog video signals was found in a 90s Linear Technology...