Need to know some steps to make a sensor

[NewToTech101]

I’m looking to make a sensor that will send a notification to an app when sensor one is too far from sensor two. I’m in square one, just looking for the right direction to go.

[straight_stewie]

5 hours ago, NewToTech101 said:

I’m looking to make a sensor that will send a notification to an app when sensor one is too far from sensor two. I’m in square one, just looking for the right direction to go.

That's potentially a tough one. I have some follow up questions:

 

• How far away can the sensors get from each other?
• Will the sensors always be in the line of site?
• How expensive can this be (what kind of components are you planning to use)?