Cheap, low-power relative distance measurement between two Arduinos

Hello. I'm trying to find a way to measure relative distance of two Arduinos. RSSI seems good since I don't need absolute distance in meters, I just want to end up with a blinking LED with its blinking frequency proportional to strength of the signal coming from the transmitter. I would need the transmitter to primarily be small (roughly, to fit in a ~5x5x2 cm box complete with microcontroller, the RF module and a battery), cheap, and, ideally, work for distances up to 100 meters without any obstacles. I should also mention I don't need to transmit actual data between the transmitter and the receiver. I have a couple of nRF24L01+ modules I've considered, but they have very basic RSSI capability with a bit flipping to 1 if signal strength exceeds -64 dBm or so. So basically I'd want something that can output an analog RSSI value. In summary, I'm open for anything that meets the following criteria:

  • small transmitter
  • cheap modules
  • works up to 100 m outdoors
  • preferably power-efficient

RSSI seems good since

But it isn't. RSSI is not a good measure of distance. Far more things have a greater impact on RSSI than distance.

I just want to end up with a blinking LED with its blinking frequency proportional to strength of the signal coming from the transmitter.

Why? What transmitter?

I'm open for anything that meets the following criteria:

small transmitter cheap modules works up to 100 m outdoors preferably power-efficient

Nothing meets all those criteria.

Ah, thank you. I didn’t realise it was that bad.
To be honest, I’m trying to find a way to locate my grandpa (who is suffering from dementia) within my grandparents’ house/yard as he sometimes wanders off during the night. If he had an RF transmitter, or at least those were the lines I was thinking along, in his pocket, sending a signal at an interval, using a receiver as a rough “hot-cold” estimate would make it easier for my grandma to find him by giving a rough estimate of the direction she should be moving in. Or so I thought. Now I’m trying to figure out something else that might work better. Maybe to have something like a “compass” that would point to the direction from which the signal of the transmitter (the components that would be with my grandpa) is coming somehow… Thank you.

Both of these particular applications are hard.

I would maybe look at Bluetooth 4.0. There is a leash function, but I don't know the range.

I also imagine there are commercial options already available.

Doing it myself with Bluetooth 4.0 seems quite difficult and expensive compared to a commerical solution. I suppose the only reasonable thing to do is to give up on the idea of doing it myself :( Many thanks for the help and quick replies.

I guess I'm missing something. If your grandfather is going to wander, and can be convinced to carry a transmitter, why does it need to be tiny and cheap? Isn't he worth more than a couple of dollars?

We don’t need this to find him, he doesn’t run away or get into trouble. Grandma can relatively easily find him, I just thought it might be a fun project, and make it a bit easier still. His dementia is bad enough that he couldn’t understand or remember what the transmitter is, and likely wouldn’t notice it in his pocket.

Here in the UK, there are commercial products combining mobile data and GPS that will automatically generate an alert if the device is taken out of it's designated home area, and can be tracked to locate the device, and can be used by the wearer to call for help. They aren't cheap, but they aren't outrageously expensive either compared to the importance of looking after a vulnerable person. (Essentially, it's a dumbed-down smartphone and a bit of custom software.)

If you were determined to make your own equivalent, a smartphone is what I'd use to do it. I don't think it would be easy to justify the make versus buy decision, though.

Indeed you are correct. If the need arises, we'll get a proper, commercial solution, but he isn't wandering away from home, just gets up and goes out for a walk around the house/yard. This was just intended as an educational project for me which would possibly make grandma's life a bit easier, far from being something my grandpa's well-being depends on.