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 can generate 10 MHz signals depending on the crystal and has excellent stability....
This TTL digital circuit from Texas Instruments documentation from the 1970s used the company's SN types...
We found this simple transistor tester in a French publication in 1981. It uses a common analog...