HF LED Power Bracelet

No problem @ODaily, glad that helped. We all learn from each other.


So had a bit of a look around.

So far the DU2805S (datasheet) seems promising, having said that I have never selected a MOSFET before just blindly used the one in whatever schematic I am following.

Yeah, I’ve never selected an RF mosfet before, but we’ll figure it out. That one has good characteristics, but I don’t know what’s up with that package.

I went through Digikey to the RF mosfet category, searched for the ubiquitous TO-220-3 package so we could try it out on a breadboard, filtered by “Active” and “in stock” and literally only came up with this one which is $22 :woozy_face:.

Looking back on this project though, Devilclarke was right, the circuit is probably ready to go, it doesn’t need a mosfet. The 2n3904 BJT can handle 200mA no problem. At least for proof of concept, that should do. Now we just need an inductor.

Used this calculator to determine that we need a 6.25μH inductor to achieve resonance at 13.56MHz with a 22pF capacitor.

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How about using a LM741CN opAmp then tune it with resistors? :slightly_smiling_face:
Like in This video.
I have no idea if it would work, but it seems like an easy route🙂

Yeah, it looks odd, it seemed to be quite a common package for RF MOSFETs. :man_shrugging:

Least the package looks how I’d expect.

Making a coil for a POC should be relatively straightforward. This should cover the details if anyone is not familiar.

I’m probably going to try order some components and tools this week, anyone got any online electronic supplies they recommend in the USA?


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Braved the quarantine to grab the original circuit from my work (miraculously still on the breadboard). Also grabbed 5.6uH and 6.8uH discrete inductors to try. I also have a shit ton of magnet wire. I’m not prioritizing this at the moment, though. Post if you figure anything out


Full quarantine hasn’t hit here yet.
Still waiting on components in the mail, so I can fruitlessly play with them. Can’t wait actually. Learning new tricks is so cool.

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This little beauty showed up today.

In a plastic case.
Inside a medium plastic bag.
Inside an ~8-9" square padded mailer.
Inside a huge padded mailer that you could stuff legal sized paper in.
Yeah, seriously.

I also got a new field detector chip. My old one escaped somehow.

I might get to play this weekend. I don’t have all the right parts, but learning isn’t always about being successful. Might just try and see what I can make happen.


It occurs to me that if a working prototype got anywhere near a scanner, that we would probably expect that scanner to not function at all.

I am NOT advocating that this be used as a prank to selectively disable the lock when that one particularly annoying co-worker tries to get in.

Just a Public Service Announcement.


Online shopping (and shipping) is an environmental disaster.

One colleague ordered a nail clipper on Amazon the other day. The thing arrived at the office in a similarly ridiculous packaging the next day. He opened it, discarded the cardboard box, plastic bag and foam peanuts, and proceeded to clip his nails.

I asked him why he didn’t buy one at the store: he replied that it was cheaper online, and it was more convenient because he didn’t have to go to the store. But get a load of this: the store is ACROSS THE STREET, and nail clippers there only cost 30 cents more.

Sometimes I think the world is walking on its head. But I’m pretty sure nature will make us humans pay for our lunacy sooner than later.

But I digress. Sorry to derail the thread…


Admittedly there are probably slightly easier ways to disable a scanner. But it is a good thing to note given the standard location for implants and the standard location for bracelets…

Do you remember those old slap bracelets in highschool, those were pretty much one size fits all and I’m pretty sure the circuitry could fit.


I think the field shape will effect the choices the most as we need it to couple well with the implant. The slap bracelet form factor would potentially work but I think they rely on spring steel to function (as in there basically metal tape measures with a pretty cover) which might mess with the field.

Actually while I remember, I was looking at the ProxLF thinking a coil in that form factor on a strap might work quite well.

Those slap bracelets, lol. Feels like 100 years ago now, but it hasn’t been that long. That’s a good idea, but it doesn’t make a complete circle around your arm. We could use a design like those, but we would have to put a flat antenna that circles around on the surface of the bracelet, which probably wouldn’t couple well with the xLED.

I was thinking more something like this, but with more turns and some kind of 3D printed spacer that we could thread the turns through. It would expand and contract a bit, kind of like a spring.

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So my xFDs showed up, and I went looking for the tracking number for the other components I needed, was all still in my cart so yay!

Something springy like that would be great, might be quite fiddly to make but sounds like it could be quite a good solution.

This would be amazing. The limited usage you get with implantable LEDs has turned me off to them, but with something like this powering them for any sort of extended period of time I would damn near buy out the store :laughing:

For clarification, would a bracelet form factor be able to reach LEDs implanted between the metacarpals of the same hand continuously/consistently (i.e constantly powering the LEDs with a relatively consistent level of brightness?).

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Why disable it at all …

Espkey :wink:

Ps the one at my past work over 4year is still there I took the liberty to let myself in to chat with one of the old security guys :smiley:

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Might need to try a cylindrical coil similar in shape to the one inside the implant. Bracelet is not working.

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Wasn’t really expecting this to work, but wanted to try it anyway.

Crystal is a AVX K50-HC1CSE13.5600MR OSCILLATOR, 13.56MHZ, SMD
Just the first one I found when I went looking.

I’d like to try it with a transistor next.

I’d like to know if it’s actually working. Funny thing is I not only have an oscilloscope, I have two of them.
And … no clue how to use them.

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