We have many versions of biofeedback with the 555 IC on this site. This is another one. We found it in October 1986 Radio Radio Electronics magazine. The power supply must be isolated.
This circuit was published in a 1979 magazine, but it can still be easily assembled, as the...
This configuration found in GE documentation from the 90s, can also be seen elsewhere in this...
This simple circuit has good gain and can be implemented with any general-purpose transistor. The...