With this circuit it is possible to vary the resistance by adjusting the mark/space reaction of the 555. The circuit was found in the November 1989 Radio Electronics magazine.
This metal detector was found in a 2001 Poptronics, but it can still be easily assembled, as the...
The inversion of the switching action, with the output going high when the input is low, can be...
The circuit shown in the figure is indicated for alternative energy sources, such as those...