How to make two sensors with range?

Question

Do it yourself

I can’t formulate a question, because incompetent on this issue. I need two sensors to make themselves known (let each have a light indicator) if both are within a radius of 20 meters from each other. How and in what way is this done, on the basis of what can this be done? The sensors themselves should be small, with a 5-ruble coin + battery.
If 20 meters is a lot, then maybe less. Just tell us what to build on and what's what in general.

Answer 1

Each sensor has an antenna + receiver. Each sensor transmits a radio signal. When a signal from another sensor is detected, the indicator lights up.
You can also try to use the sound by replacing the emitter / receivers with a speaker / microphone, respectively.
Regarding the specific implementation - google Chinese sensors / bugs / etc. Perhaps there are even completely ready-made solutions, I can’t tell you. I myself am not aware.

Answer 2

BLE tag is called. Works via Bluetooth, broadcasts ID.