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.
We found this circuit in a 1990 documentation. The operational amplifiers are type 741 and the LED is...
We found this circuit on an Elektor from the 90s. The frequency is given by capacitor C1. The...
Found in National Semiconductor publication from the 1980s. This gain-100 circuit can be...