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.
Quadrature Generator for Transistor and SCR Testing This circuit was found in US 1980s...
This circuit, with output currents up to 500 mA, can stabilize or regulate the output voltage of a...
This circuit that uses only transistors that are still common was found in a 1995 documentation. The TIP41...