Ask Hackaday: Can Battery-Free Bluetooth Item Locating Tags Exist?

iFind Tag

[Vishak] tipped us about the iFind Kickstarter campaign, a 1.25×1.06×0.09″ (32x27x2.4mm) tag meant to be attached to anything you may lose in your daily life. This device communicates with Bluetooth Low Energy (BLE) enabled smartphones, has a 200ft (60m) detection range and a loud alarm. What is interesting to mention is that this device doesn’t need any battery to operate as it

recycles electromagnetic energy and stores it in a unique power bank.

As you can guess, this particular claim intrigued the Hackaday team given that we never featured so small energy harvesting devices. The ‘closest’ thing that comes to our minds is the Allsee project, a simple gesture recognition device that uses existing wireless signals (TV and RFID transmissions) to extract any movement that occur in front of it. However the antenna was quite big and very little power was extracted.

A quick Google search let us know that Bluetooth Low Energy solutions usually consume an idle current of around 10uA @ ~3V. The (very) successful Sticknfind campaign which promoted the same battery-enabled product claimed a one year autonomy with a CR2016 battery and a 100ft range, leading to a ~90mAh/24/30.5/12 = 10.2uA idle current. As we’re not expert on the subject, we would like to ask our readers if they ever came across such energy harvesting performances (3V*10.2uA = 30uW) in a normal home environment. Our very bad maths indicate that if one would like to extract power from a typical Wifi router located 2 meters from you emitting 0.5Watts of power (in a perfect vacuum environment) with a 32*27mm = 864mm = 0.000864m² tag you’d only be able to get 0.5 * (0.000864/(4*pi*2*2)) = 8.6uW.

It is therefore too bad that we can’t see in the presentation video what is inside the iFind, nor more details about the patent pending technologies involved. We hope that our dear readers will enlighten us in the comments section below.

220 thoughts on “Ask Hackaday: Can Battery-Free Bluetooth Item Locating Tags Exist?

  1. In my mind, the biggest red flag is this:
    If they can really achieve what they claim, this is a revolutionary technology that could easily attract venture capital or even be financed via a partnership with a future licensee.
    In either of the cases I mention, they could simply prove their technology by demonstration and receive financing without the hassle of a Kickstarter campaign.
    They would certainly not need to resort to crowdfunding the project.. The fact that they are using Kickstarter raises flags, in my opinion.

    1. Of course, thats on the assumption that the VC’s don’t know how to use a browser to see the many similar examples which have already been placed in the public domain re EM energy harvesting.
      Also, without a patent the VC’s wouldn’t touch it with a barge pole. And without ‘novelty’ they wouldn’t be able to get a patent…I suspect novelty has been lost as there are so many examples already in the public domain as prior art. If their device is novel and not considered obvious to someone in the ‘art’, and they have filed a patent application, then they are doing the right thing by not telling the world how it works on a website.

  2. Could a piezoelectric beeper also work as a device to turn ambient vibrations into power?

    Put these on things that get moved around a lot and there should be plenty of vibration of the beeper.

  3. I’ve gotta call “mostly bullshit” on this one. While none of the claims, in itself, is technically infeasible, doing it in anywhere near the volume and unit cost shown is. So far I’ve heard claimed: BLE, ambient RF harvesting, shake-to-charge, active modes (“rope”), internal buzzer, storage for several days’ worth of energy, accelerometer… anything I’ve missed?

    Let’s start with RF energy harvesting. They are claiming it harvests ambient signals from “Wi-Fi transmitters, cell-phone antennas, and TV masts”. This is plausable in the uW range (assuming you’re reasonably near a strong transmitter), and has been done. Several have made the rounds recently, using e.g. backscatter modulation on ATSC television signals to act as passive RFID-type transponders or positioning sensors. The RF frontend is invariably based on the WISP design (, featuring an antenna and matching network well-tuned for a specific band (for decent results, a specific station at that), followed by a long charge-pump ladder with very low-Vf Schottky diodes. Assuming you can get at least some couple hundred mV at the antenna output (to overcome the diode drops), you can slowly charge the output up to the 2-3 volts needed for small microcontrollers and transmitters. The big fails here are antenna size and broadband-ness. A dipole matched to 2.4GHz is a few inches long. For TV and most cellular signals, considerably longer. This could be marginally worked around (with losses) by massively folding the antenna or funky designs (novel multiband fractal antennas), but the types of people who can pull the latter off don’t need to beg for money on Kickstarter. The tuning bit is a killer; this needs a high Q matching network to get the voltage, and that means high selectivity. A broadband harvesting implementation does not seem feasible.

    Don’t forget that this supposed BLE antenna (also 2.4GHz range) has to be in there, and without detuning the supposed multiband harvesting antenna(s) and vice versa. Again, the person who has both working concurrently on the SAME antenna doesn’t need Kickstarter.

    The power storage mechanism: Supercaps have gotten decent and are now available in thin form factors (Cap-XX, and graphene-based devices which are not yet on the market commercially)… more likely, thin-film battery. I have to assume this device will be passively detecting Tx packets (BLE or otherwise) rather than sending out continuous pings of its own, and (ab)using BLE advertising packets as the response mechanism to avoid actual association overhead. I have to make this assumption to make the “several days” of charge in RF-denied environments claim plausible. Just not in the volume shown, unless it is the only thing in the package. The main limiter is not even so much energy density, but power density (the peak bursts of even a few mA will tax a tiny thinfilm battery, and if these devices need to work outside in winter, tough noogies)

    The shake-to-charge mechanism: My day-job is at a company that makes (among other things) piezo energy harvesting devices. While these exist and work, there’s just not enough volume in the package shown for one to generate any usable amount of power. Typical device is a disc or cantilevered beam (diving board) with a mass. When shaken, the deflection of the piezo by the mass, in opposition to the clamped surface generates a small amount of charge. There are many piezoelectric materials, but the only really cost-effective ones for this are PZT and PVDF. For this to produce usable amounts of power requires (at minimum) 3 things:

    a) Sufficient piezo material to strain
    b) Sufficient distance to strain it
    c) Sufficient length and mass to tune it

    The device shown fails on all counts. Per above comments, the injection-molded case leaves ~1mm internal volume to be divided among electronics, energy storage and piezo harvester. If they are claiming to have RF harvesting, BLE, a buzzer and an accelerometer, they’ve already used up all of that (and then some). Even if that area were all piezo, it’s just not enough material to generate appreciable power with a few shakes. There is also not enough length for the material and mass to deflect into to generate the strain (ruling out PVDF entirely – it needs to move a lot), nor to tune it toward usefully harvesting the single-digit Hz you’d get from shaking by hand (I’d estimate the natural frequency of an unmassed–it won’t fit–PZT wafer in that envelope to be in the 500 ~ some kHz range – it’s not even going to know you are shaking it. Maybe if you bang it against a hard surface repeatedly…).

    TL;DR: Each individual claim they make is technically plausible on its own, for a device with a MUCH larger form factor. It could maybe still fit in your pocket, but you wouldn’t want it on your key ring. And cost is another story. The whole thing strikes me as a case of overly optimistic students or enthusiasts who have “done the homework” on paper (i.e. seen each of these technologies working, individually), but have very limited practical electronics backing. I suspect they (and customers!) will be in for a very rude awakening when they go to manufacture this. Well, hopefully that, and not charlatans out to make a quick buck.

  4. I got bored with the RF generator idea about halfway down the page, but has anyone considered tritium power? 12.3 yr half-life, and possible that even at 50% capacity, it’s still producing enough (harmless) radiation to power a locating transmitter. I’ve seen tritium flashlights, and was looking into the possibility of tritium cell phone batteries, though, currently they’re way too expensive and underpowered for even an old nokia. Still, never having to recharge your phone again, it just runs for 12.3 years before dying… could be cool.

  5. They’re fools more than scammers at a guess, but either way this project is a bunch of crap. I mean look at their graphs on They draw some context-free meaningless graphs and then start talking…

    “When the input power is 10dBm, which is typical for home wifi”


    That’s 10mw (=10dbm) *output power of the frickin’ transmitter*. Errr… Inverse square law anyone? Omnidirection antennas? … Bueller? ..Bueller?

    Ok for example; I just checked the signal between my two home wifi routers, with moderately decent antennas, about 40ft apart in open space; the receiver end is showing RSSI of -67dBm, that’s 1nanowatt.

    …Maybe if you had an illegally powerful wifi transmitter hooked up to a Yagi and you stick-taped their device to the end… juuuust maybe… eh, nope not even then.

    My verdict: comically inept, borderline fraudulent.

    1. DrTune,
      I agree 100%. I have a “max” signal to my wifi router and my RSSI is -47dbm. Even at 100% harvesting efficiency, the amount of energy harvested would be several orders of magnitude lower than what would be required for the tag to simply function as advertised, much less charge it’s “power bank”. It’s hard to even imagine an EM environment where the tag could accumulate a charge. I don’t think I would want to hang out in that environment for any length of time.

  6. I am a firm believer this is a scam. I’m compiling a Google Doc which includes some quotes from here. If you’d like some amusement, or see errors/omissions of substance, pls leave a comment here and I’ll add/correct.
    Thankyou, and happy hacking.

  7. the math ain’t right because your assumption that there is such a thing as a perfect vacuum. Which is why the professional class will be proven to be just book smart. Statements that are written down, thus recorded, in ‘scientific’ media are regurgitated as the only proven facts, but to an enlightened person, the see the mistake of not proving the measurements were done in the stated environments like a perfect vacuum or those that are free from any magnetic, electric, gravitational, or other forces… 0 divided by 0 = ? prove it in the real world by showing how the absence of nothing can be divided/split/separated and then reconnected thus = 0 power transmitted/absorbed thru/from/into space/air is only half of the circuit!

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.