Keychain finder nrf24l01

Hello, i have an idea to make 8 keychains (5x5 cm) for сhristmas gift, that can detect each other when near.
So i want give my friend 1 keychain, and i have 1, when friend arrive near me (like 100m or smthn) the keychain will blink led for 3 sec, and maybe play some short shit melody.
I dont know how to do that, i think it can be nrf24l01 (because i have a lot) and some arduino chip. It need to be powered by something, like 3.3v computer motherboard battery.
I'll be grateful for your help, guys.

For this to work, the senders will need to be constantly sending out pulses to be detected by the receiver. The receiver will also have to be constantly on drawing some 50 mA.

Battery life under these conditions would be in the order of hours.


Thx for your help!
So first i need to change battery to something like Nokia BL-5C, i think it will be good enough.
Then i need to add 1 minute pulse interval to save some power.
What controller will be best for it? ATTiny85, Atmega328p or atmega8?
I think ATtiny85 will be best for it, what do you think?

Then i need to add 1 minute pulse interval to save some power.

Alas, it is not that simple.

What is essential is that all the devices are listening and transmitting in the same time window. With a gap of a minute it would be pretty much certain that they would NEVER be in sync.

You also need an interval that is appropriate compared to the movement of the devices. The two wearers could cover a lot of ground in a minute and never know they had been close to each other for part of that time.

If all of the devices can listen ALL of the time then that gives more flexibility about how often signals are transmitted. But if two devices accidentally transmit at the same time the signal will be garbled and will need to be repeated - but perhaps not immediately. However, without an acknowledgment system the transmitter will not know its message was not received. And, just to make things even more complex, you have described a system in which the absence of a device in range (and hence the absence of an acknowledgment) would be normal.

I guess you could give each device a different repeat interval carefully chosen so that even if they overlap once they will not do so next time. But that would need some careful thought when there are several devices.

And 100m might be stretching the capability of NRF24 devices unless they have carefully oriented antennas and are out in the open.


Hmm, thank you Robin, now i understand my mistakes and working principle of this device better.
Range isnt so important, it will be good if it works 30-50 meters.
Its just a keychain, so i want to do it cheap and simple, so if u know better way to do it, maybe it will be bluetooth, or something, let me know your opinion please.

so if u know better way to do it,

Sorry, but I don't. Maybe someone else will.

I think it is one of those ideas that seems simple until you study it carefully.