Connection between two hidden NRF51822 modules

[vldzero]

Hello

First of all, I would like to apologize if a similar question was already asked but all this BLE matter is so broad that it defeats me. I am planning to put two NRF51822 devices into my project. I want one of the devices to pull one of the GPIO pins down as soon as the other one is in the range of 10-30m. Also, I want it to be secure, I don't want anyone else to scan my devices, copy them and control the pin with another one. I want slave device to consume as low energy as possible.

My idea is:

1. make both devices undiscoverable so it will be difficult to scan for them.
2. The master device (the one that controls the GPIO pin) will advertise only to the slave one and it will be visible only for the slave one.
3. The slave device will check if it is in range of 10-30-m and if so it will send back a message/password or whatever to master. If it is not in range it won't send any message.
4. If master receives the message it pulls the pin down.
5. Repeat points 2-4 each 1s.

Is it possible to implement such routine? Or are there any better ideas? I am a total newbie in BLE. I managed to run simple advertising example but nothing more. I would appreciate a guidance, not a readymade solution.

vldzero

[Petter Myhre]

I'm not sure what you mean by undiscoverable, the peripheral/slave has to advertise for the central/master to know that it is there.

The central/master doesn't advertise, it scans, the peripheral/slave advertise.

You can use a private resolve address in the peripheral to enforce privacy. Then if it bonds with the central only the central will be able to resolve the address and recognize the peripheral.

What you need to figure out is how to pair. Can you pair these devices in a safe environment? Do they have any inputs or output? Can they use a preshared key? It is a bit difficult to recommend something when I know so little about your application, maybe you can explain a bit more?

The only option for measuring estimating the range is by using RSSI, Received Signal Strength Indication, so you can look into that. Many questions in here about that.