Advice on feasability of bluetooth project.

I am moving on to my second project and I would like to create a pair of devices that will alert me somehow when they move x distance away from each other. I worry about losing my wallet so, the idea was that i can make a small device that is with my wallet and another on a key chain with my keys. If i lose/forget/drop my wallet, once i move, say 30 metres away the device on my keys will beep/flash or maybe my bluetooth enabled android smart phone will alert me.

The wallet device needs to be small and quite flat. Small enough that I can still fit my wallet in my pocket with it inside.

The device for the key obviously needs to also be small.

I have considered a few options but due to size restrictions, range requirements and cost bluetooth seems to be my only option for this. My idea is to use 2 bluetooth RF serial transceivers such as this:

http://dx.com/p/hc-05-wireless-bluetooth-serial-pass-through-module-for-arduino-green-240417?tc=GBP&gclid=CMC9gLOVpboCFbPItAodlh0AKQ

and a 2 very small arduino clones such as this to drive the serial communications:

http://www.femtoduino.com/spex/femtoduino

I hope to have it set up so the wallet device sends an intermittant signal, say once every minute, to the key device and if the key device does not receive a signal it will alert me.

I think what I would mainly like to know is how small do you think i can shrink one of these devices down to, ideally i would like the wallet device to have overall width and height of a credit card and as slim as maybe 3-4 credit cards stacked .Is this a possibilty or am i dreaming here. I worry about getting it thin enough and if i do how robust is it going to be?

Also, power is my next issue. The only way i can image is with watch batteries. I have yet to look into this much but do you think they will provide enough current for the devices? If so what can i expect with battery life if I can utilize the sleep modes etc? Can I use the rechargeable watch batteries? If so, it would be cool to have a wireless charging mat where they sit while im at home.

So, what do you think? Is this doable? Do i have any other (relatively cheap) options rather than bluetooth? should i be thinking of building my own atmega based boards rather than using the small clones? What are the pitfalls you may expect to come up against? I would love to hear from your experiences with bluetooth, small builds and low power situations.

Thanks

Loz

After looking a bit more i have found these:

also
http://www.ebay.co.uk/itm/SLBoat-BLE-CC2541-Mini-Blue-Module-/171115402973?pt=US_Radio_Comm_Antennas&hash=item27d745d6dd

These look perfect for this. Do you think I can utilize these with this project? I'm guessing i will need to also add an antenna to each?

All the bluetooth modules you show have the antenna built-in. You would need to ensure that at least one of them can work as a master, like the HC-05. An advantage of bluetooth is that the assembly is about 3mm thick. You might also consider the NRF24. It would be about 50% thicker but they have about a third of the power consumption. while the range is about the same.

Thanks. I’ve just had a quick look at these and found this at £2.56 each, also only 3mm thick:

Seems like a bargain. The only worry i have with them is they come with the header pins soldered on. Trying to remove them on such a tiny board seems a bit of challenge but @ £2.56 it’s probably worth a go.

I noticed with the Bluetooth modules that they have a feature call RSSI which can pick up on signal strength. I was hoping to use this to to help code at specific ranges < 100m. Do the nrf24l01’s have a similar feature?

Nick_Pyner:
You would need to ensure that at least one of them can work as a master, like the HC-05.

How do i know from listed specification if it can operated as a master. Does this apply to the nrf24l01’s also?

loz2111:
The only worry i have with them is they come with the header pins soldered on. Trying to remove them on such a tiny board seems a bit of challenge but @ £2.56 it's probably worth a go.

Correct, but, if that worries you, soldering the close SMD pads on the bluetooth modules will too.

I noticed with the Bluetooth modules that they have a feature call RSSI which can pick up on signal strength. I was hoping to use this to to help code at specific ranges < 100m. Do the nrf24l01's have a similar feature?

I think this might be a bit fanciful at 100m. I understand RSSI is pretty crude and works like "quite close", "very close, and "inches away" and it will probably be lying if there is a wall in between. 100m usually qualifies as "out of range". I don't think NRF24s have this feature.

How do i know from listed specification if it can operated as a master. Does this apply to the nrf24l01's also?

There should be a configuration note - master/slave or slave only. This does not apply to NRF24s.

Well thanks, your advice was really helpful. I went for NRF2L01 modules and managed to find some headerless ones for £2 each:

http://dx.com/p/lc-nrf24l01-wireless-data-transmission-module-blue-234534

Are these good to use for my application?

Now to try and decrease transmitting range. If I decrease supply voltage will that decrease transmitting power, or is it not that simple? I don't have access/can't find a datasheet for the modules but it say in the spec that they take a supply voltage of 1.9v - 3.6v and i I can not see a voltage regulator in the picture.

Data is available from Nordic Semiconductor.

I can't comment on the efficacy other than than the modules you refer to have the same components and connection as the typical ones with headers, and will probably work the same. I think I understand what you are trying to do in the matter of range, I don't think doing so is as simple as you propose. I imagine that keeping the range you have reliable is also important as I understand these things are only good for about 10m anyway - about the same as bluetooth.

Note that you can get a high-power NRF24 module with an external antenna. This is good for about 100m.

I have some NRF24s but I have never used them. Bluetooth suffices.

Nick_Pyner:
I imagine that keeping the range you have reliable is also important as I understand these things are only good for about 10m anyway - about the same as bluetooth.

Bu that is exactly what he wants - he is not at all concerned about reliable data transfer, rather to know when communication has failed!

Clearly there is no way of using any sub-microwave radio system to range-find accurately over such short distances, but using a weak transmission and noting when it fails is not such a bad approximation. It is just as relevant to be notified when there is a wall between the wallet and yourself, as a stretch of open space.