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 is from a 1965 Motorola manual. It can be implemented with a common 2N2646 and a 106...
This 1967 Electronics World circuit is suitable for frequencies up to 1 MHz. The setting applies...
T1 and T2 can be BC557 and T3 a TIP32 for lamps up to 3 A. The supply is made by voltage of 12 V...