yes, it is 3.2 mm thick, vs an ordinary ccard that is about 0.8mm thick - so the thickness of FOUR credit cards, not exactly meeting the no-bulge goal

About the thickness (maybe a smidge more) of a typical door/id badge, though

Passive BLE is a thing that's in active development (meaning it wouldn't require a power source) – this has the potential to work, but the range would likely be extremely limited. Inches, not feet.

The right design would have the ability to reply to another device with an advertising beacon, which could work in a limited set of circumstances, with some tweaks to how the AirTag protocol currently works. Like, if you stuck your wallet in your pocket next to your phone or on a table next to your laptop.

RF ambient energy harvesters also exist, which use more generalized RF to gather energy in a generic way. Where RFID sends the signal intentionally that powers the RFID reply beacon, you could theoretically find something in the ISM that gives you enough power to somewhat reliably harvest that instead.

https://www.digikey.com/en/articles/rf-energy-harvesting-bat...

Not sure about the advertising beacon interval but if it's flexible enough you might be able to join the two concepts. The limitation is likely figuring out the right ISM band, and the right package (coil size) to harvest enough energy reliably to send out your advertising beacons.

Distance and directionality (phase) once again affects energy output, but also, you would need to build a coil to a specific size to the frequency you're trying to harvest. Not sure these ever match up for this application. Higher frequencies would allow for smaller coils, but with lower power output efficiency.

The math behind this is wrapped up in the definition of the Q factor, and this is very similar to how you understand radio propagation in general, if you're looking for something to research further.

But, my ELI5 to myself: BLE beacons are already using ISM, so if your goal is to retransmit an ISM signal that beats the noise floor you're using to harvest RF in the same range, efficiency is going to have to be phenomenal in your system design (or take advantage of some power differential).

There's been some interesting projects and papers on this in the last 10 years or so, and the UW had an interesting retransmission concept a year or two ago they built. Links appear to be broken now though.

> But, my ELI5 to myself: BLE beacons are already using ISM, so if your goal is to retransmit an ISM signal that beats the noise floor you're using to harvest RF in the same range, efficiency is going to have to be phenomenal in your system design (or take advantage of some power differential).

Harvest in one range (ex: 5.8GHz) broadcast into another (ex: 2.4GHz)

Keep in mind that the chip is only the chip. You will also need casing and an electricity source, so won't be as thin as it might look at first

Ultra thin LiPos exist, a quick search found some as thin as 0.4mm. However I would imagine that packaging for product safety is a concern, as you’d also need to appropriately shield the cell from damage.

And the use case—a wallet that users are likely to sit on—is not the easiest in which to make a safe ultra-thin battery...

Normal airtags don't use rechargable lipos - they use a CR2032 coin cell battery

I know. I am entertaining the OP's exercise in how one might make a thinner device. CR2032s are more than 4 times as thick as a credit card, even without a socket or a housing.

The thinnest commonly available coin cells are 1.2mm thick, which is better, but still will require a creative interpretation of "credit card size".

An ISO credit card is 0.76mm thick.

How are you charging a LiPo in a CC sized device? How are you protecting it from the stresses of a wallet? Final problem with this - they'd give you 20-30Mah of power, which is nowhere near what you'd need for a decent battery life in one of these.

tldr; we're not there yet...

The easiest way to charge something thin would be a proprietary pogo-pin adapter and pads.

Protecting it would certainly be a challenge. If I were to try, my first attempt would be to sandwich it between two sheets of stainless. (0.76-0.4)/2 only leaves .18mm left over... that's 33 gauge. Maybe too thin to add enough rigidity? If you used 28 gauge stainless that would bring the sandwich to 1.04mm, which might be a better compromise.

It would certainly need some prototyping.

Apple gets 1 year of battery life from 225mah. So, it seems reasonable to get a month from 30mah. Not great, but hey, we're just entertaining the packaging requirement.

This would be good for bicycles (ie an airtag that doesn't look like an airtag)

Bicycle thieves know to look for and remove airtags now, I would definitely pay good money for something more covert, that looked like it was just a part of the bike.

Doesn’t the AirTag start making beeping sounds to alert someone they’re being tracked because Apple doesn’t want it to be used by stalkers? Even if a third-party makes a tag which is silent, would the thief’s iPhone notify them there’s an unknown tracker moving with them? It would defeat the whole point of using them on bikes, etc to catch thieves and recover stolen goods.

Android apps that detect AirTags exist too, e.g. https://github.com/seemoo-lab/AirGuard

They do, but people have of course realized that you can just destroy the speaker and then it won't.

But if you have a phone with U1 chip it will give you precision finding of unknown AirTags, making them a lot easier to locate than a bluetooth beacon.

See vanmoof.com Their bikes use the findmy network

definitely. I really only buy crappy bikes for that reason right now... a solid tracking option would do wonders

I believe this already exists

https://chipolo.net/en-us/products/chipolo-card-spot

Unfortunately there are 2 serious drawback to this: 1. Non replaceable/rechargeable battery 2. No support for directional tracking using U1. It’s just the standard find my networks Bluetooth beacon and the ability to emit a sound. Not the local directional finding available with AirTags.

> 1. Non replaceable/rechargeable battery

How practical/possible would it be get get Qi wireless charging into this form factor?

That doesn't use U1 or U1-like tracking, though (can only beep, no precise finding like AirTag).

yes, it is 2.4 mm thick, vs an ordinary ccard that is about 0.8mm thick - so the thickness of three credit cards, not exactly meeting the no-bulge goal

You're not going to fit a batter of any useful size, along with enough protection for said batter inside a .76mm thick device today. A .4mm liPo (which I'm not sure I'd want bending 100x a day in my back pocket) has maybe 30Mah of capacity - that's just over 10% of a cr2032 (what AirTags use). 1 month battery life is not what I'd consider useful in such a device.

Just because you can get a chip that fits in there, does't mean you can build a product...

Hrm, what about a thin film supercapacitor that's charged by a piezoelectric component fueled by the regular flexing?

The part you mention is 0.56mm thick including solder balls (0.37mm nominal without), and a credit card is 0.8mm thick. The thinnest 'normal' PCB is 0.4mm. Add some thickness from the exterior packaging, and the product will be the thickness of two credit cards, at least. You'd need to use a wafer-chip (with no package) to get to credit card thickness, then add a very thin battery of some sort. This is a very challenging and expensive project.

If you are willing to tinker and solder a bit, you can make your own card sized AirTag pretty easily:

https://youtu.be/7rHyAAkf5tE

Nice. He has a v2 that doubles the battery and got speakers working https://www.youtube.com/watch?v=Dze4L1YuJYA

Doesn't pass the no-bulge requirement, but I've carried an AirTag in the coin pocket of my Big SKinny wallet for several months. Granted, the wallet sits in a side pocket on my pants, and not in the back pocket.

I've already had to track it a couple times.

I find the official Apple AirTags to be woefully low-volume. Would love to buy a protocol-compatible third party "AirTag" which has high volume, especially if it could also be a bit thinner.

The biggest drawback to the AirTag design is that the plastic housing is the speaker cone, so when they're against another object, it damps the sound.

The Chipolo CARD Spot is an example of such a product.

https://chipolo.net/en-us/products/chipolo-card-spot?cl=head...

Also, the Tile Slim tracker is very comparable:

https://www.tile.com/product/black-slim?pack-size=1&category...

It doesn't implement the U1 protocol, rather it's a Bluetooth LE-based tracker that works with any phone, but it fits your form factor. Buy one and peel it apart, or look at their FCC filing photos:

https://fccid.io/2ABXLT1601S

https://fccid.io/2ABXLT1601S/Internal-Photos/2ABXLT1601S-Int...

or a teardown:

https://youtu.be/Vl6EdSlNKQo?t=61

It uses flexible PCBs, thin rigid PCBs, and an extremely thin battery like this one:

https://www.ultralifecorporation.com/default.asp?LINKNAME=LI...

plus the processors, piezo buzzer, antenna, as well as injection-molded plastic shell.

To build this from scratch, you need an electrical engineer, a programmer, a manufacturing engineer, a PCB manufacturer, and an assembly manufacturer. Be aware that hardware startups are hard, you can't just throw up a webserver and scale later, you are probably needing at least 800 hours of skilled engineering effort, probably double that if you're new to electronics manufacturing, just to get a comedy-size PCB conglomeration which would get you tackled by security the airport, might have 3 days of battery life, costs $200 each, and can be found by an iPhone using the Find My network. Then you have to miniaturize it and get the BOM cost under $15 (which won't happen at quantities under 1,000) and the assembly tooling semi-automated so you can build it in under 120 seconds of hands-on operator time and then you can sell it for $29.99, until someone on Aliexpress rips off your design and sells it for $24.99. See https://www.ycombinator.com/library/47-product-advice-for-ha... for some less bitter advice.

How would you power it? Do batteries that thin exist?