RSSI and AoA for tiny consumer micro-locator ?

[Visions]

Project Overview:
ultra-compact microlocator device that can be attached to items like glasses and located using a phone app. Key requirements:

Form factor under 2mm thick and 6mm diameter
Long battery life
BLE 5.3 for connectivity and location

Components:

IN100 Bluetooth SoC (InPlay Technologies)

DFN8 (2.5x2.5mm) package
BLE 5.3
Ultra-low power, less than 10nA lost in sleep mode


V335 Coin Cell Battery

1.2mm thick, 5.8mm diameter
Should enable multi-year battery life


26MHz Crystal

Required for IN100's RF operation
Small 2.0x2.5mm package


Antenna

WLA.01
Required for IN100's RF operation
Small 1.6x3.2mm

Question: Am I missing something? It seems as though this setup would work for sub-meter location first using RSSI data to get in the ball park and then AoA when close . I would love to have a rhinestone that I could glue to my glasses and never lose them!

 

[rjenkinsgb]

An interesting project! Nice concept for both individual item location & home automation.


The battery size/type is confused though - you first list a lithium cell. CR1225 which is 12mm diameter by 2.5mm thick.

I see "V355" in a drawing, but the data sheet is for a 335 / SR512SW.


That is a problem.. You do not appear to have allowed for the transmit current, ~9 to 14mA (data sheet section 5.5).

The SR512SW is not really intended for high loads; the "high current" pulse test in the datasheet uses a 2K load, so around 0.6mA (compared to around 9uA for normal discharge).

That 9+ mA is likely to kill the battery and reset the IC, either instantly or quite early on, compared to the expected life if the current was low.

It would need a fairly large capacitor to buffer the IC current, at least, if it could work at all with that cell.

That's a "low drain" cell type, according to Renata - they categorise cell types as low or high drain, plus special high pulse types (not listed in that data); see here:

Silver Oxide batteries | Renata SA

www.renata.com

 

[Visions]

Thank you! Yeah I had that confused from a previous project. So then I would have to go slightly wider, slightly thicker or both but either of these should work right?

 

[rjenkinsgb]

Those are still both in the "Low drain" class rather than high drain; the 361 or 376 are fairly small, in the high drain class?
- But, the data does not give any info on peak current capabilities for any of those.


I think you need to contact Renata technical support to find out from them what cells can support the peak current the transmitter needs, without compromising the battery life too much.

 

[Visions]

Thank you! I'll give them A call. I think low friends is what I'm looking for since I want the battery to last for multiply years but like you pointed out there are those spikes that it needs to be able to handle as well.

 

[Nigel Goodwin]

A common technique for long life batteries is to add a super capacitor across it, with the super capacitor providing the high current pulses which the long life batteries are unable to supply.

However, I suspect the tiny size requirement might preclude that?.

 

[danadak]

Consider modeling, simulation, the combo of super cap and battery, as the
super cap, for small valued caps, has fairly high Z and so its effectiveness
at handling pulses may not be a good solution. Depends on your tolerance
for magnitude of transient V drop. Looking at a 2032 coin cell itsZ ~ 20 ohms,
the smallest supercap 60 ohms. It helps but not all that great.

Pulse-powering heavy loads with a coin cell

Lithium coin cells are rated for fairly low standard current draws, on the order of 1 to 5 mA. Also, while they allow greater pulsed current draws (i.e., periodic bursts), this appears to be detrim...

electronics.stackexchange.com

Regards, Dana.