Where can I find the circuit for the Magnetic Field Exit Sensor?

Question

Sensors

Dear gurus, the crux of the matter is as follows. Element "A" creates around itself a field with a radius of 40 cm. At the moment of going beyond this field, element "B" starts blinking. The number of elements "B" is at least 2x.
In terms of dimensions, I would like to create something pocket-sized. Where can you find a diagram? Please share your knowledge and experience. Appreciated in advance. E14

Answer 1

It's not entirely clear what this is about. What is A? What B? Why blink?
Well, there is a reed switch - a door opening sensor in any alarm system. The magnet is close - the reed switch is closed, far - open.
There is a Hall sensor - in any modern car, magnets spin on the camshaft, their position is read by the Hall sensor. The keyboards also used to stand.
There are keywords - then Google to the rescue!

Answer 2

Andrey, thanks for the answer. "A" creates a field. "B" - sensor with battery and bulb. The reed switch is not suitable. "B" can change its position in all planes, but it will turn on only when the created "A" is moved outside the field.