solution for power on a circuit board when it detects a magnetic field.

Projects General Guidance
1

I have an arduino based circuit that does some mechanical job. I wanted that the circuit is turned on when it detects magnetic field. I want to use a magnet as a reader for the circuit. When a magnet fitted in something say a stick is put near to circuit board, it detect magnetic field and turned on. I learned an operational amplifier can be used for this purpose.

I need guidance for how to do it. I am a novice in embedded systems & electronics.

2

samFin:
I learned an operational amplifier can be used for this purpose.

It might be a lot simpler for you to start with a magnetic reed switch. Here's a Mouser search listing with a bunch of reed switches so you can learn about them.