This very high input impedance JFET circuit was found in the 1995 Encyclopedia of Electronic Circuits Volume 5. The JFET is a general-purpose transistor as is the output transistor.
With the configuration shown in the figure, we have the relay being activated when the microphone picks...
We found this circuit in a 1992 Poptronics. What it does is to activate LEDs sequentially,...
In the figure we have an application of a voltage comparator in a circuit that fires with the...