This indicator or logic probe was found in the April 1979 Radio Electronics magazine. The integrated circuit is the 555 and the FET can be any general-purpose type.
Using components from the era, this 90's EDN circuit is used in analog to digital conversion...
Found in an American publication from the 90s, this circuit allows the supply of 3, 6 and 9 V...
We found this circuit in an old Popular Electronics. The circuit is experimental, driving an LED...