This circuit was found in ancient American documentation. It makes use of the operational amplifiers of the time, but it can be implemented with modern equivalents.
This is an interesting configuration obtained in an old circuit manual. The unijunction can be...
We have many versions of biofeedback with the 555 IC on this site. This is another one. We found it...
We have in the figure as a monostable a proximity switch where our body capacitance (estimated at 200...