Allow 12v power through when RF transmitter is close

Question

I'm currently working on a project to allow 12v power through when a RF Transmitter is within a 20 foot range and shuts off then the transmitter is out of range, I'm not so concerned about the range being accurate just needs to be approximately 15-50 foot away from the receiver.

I found this https://www.maximintegrated.com/en/design/technical-documents/app-notes/5/5099.html

But this is allowing 3v to flow when the RF is out of range instead of when its in range,

How could I switch this set up to be reverse. or is there a better approach that I'm not thinking of.

Sorry if this is an easy fix but I'm just getting into circuits.

Answer 1

MAX9075 is a comparator, and its two inputs (marked + and -) can be swapped to invert the logic.

Note that the comparator is not suitable to drive a 12 V load directly - you should instead use a MOSFET as a switch driven by the output of the comparator instead. You can use your 12 V supply as input power to the whole system (including the load switched by the MOSFET), and use a 3 V regulator to provide 3 V to your LNA, power detector, and comparator.

Note that this isn't specifically sensitive to your transmitter, just any transmitter with a close-enough frequency.