How to track the frequency of the brain at home?

Question

Electronics

Example: a device is put on the head that constantly reads the frequency with the ability to see it in numbers and on a graph from a smartphone or computer, while the person relaxes and begins to meditate. If he does it right, the frequency will drop and it will be clearly visible.

What of these devices already exists and is available to everyone (does not require kilodollar dollar investments)?

Answer 1

So you take it and track it.
There are even shields for arduino on aliexpress to read brain waves.
The software also googles.