How to make a brainwave sensor and circuit device

The principles of EEG and ECG devices are actually similar. The main difference is in the location of the probe and analyzing software. Bookstores sell books on the principles of several medical devices, the circuit diagrams are available, and the probes are usually bought off-the-shelf. Some manufacturers have produced several new amplifiers specialized for medical devices, and the data and application manuals explain the circuit principles. You can look up Analog Devices, Microchip, Cypress, etc. on the internet.