Short range but wide area antenna design?

I’m looking for a clever way to lock my Windows PC rightaway when I leave my desk. Most of the times I remember to hit Win-L, but sometimes I don’t. I did set a 2-minute timeout, but my workmate in the office next door noticed and immediately rushes to my office to flip my screen or other such stupid jokes when I’m clear away.

So, aside from getting mad at him - which I’m not gonna do, because I like him and it’s a kind of game we play, the obvious solution is to install a switch on my desk chair (which is spring loaded): take the weight off the chair, switch goes off, PC locks. But that’s too easy and not very grinder-like.

So I have a better idea: weave a copper wire into the fabric of my chair to form a flat coil antenna, and implant a transponder in my butt. Not only will it lock the computer when I get off the chair, it’ll also unlock it immediately when I sit back down. I won’t even have to present my hand to the reader under my desk anymore. Genius!

Trouble is, I don’t always sit exactly at the same place all the time. So the antenna would have to be a lot more forgiving with transponder placement than regular readers. Ideally, I’d like to reuse the electronics of some existing el-cheapo USB reader too, so I don’t have to roll my own. But I have a feeling a wide antenna will make a wider, but also weaker field, and that won’t work so well.

Anybody has good pointers on RFID antenna designs for that kind of requirements (as I’d probably go LF to increase chances of this working)? I’ve googled around, but nothing very useful came up…

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Sounds like a job for a UHF tag (not implanted.)

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Butt transponders. We really are at the frontier of science.


I think they might have something like that on the GitHub Suppository …


Do you have pictures you can share of your chair or work area? This honestly should be a relatively easy thing to work out, only complication would be working with an xEm read range. I mean you could buy an Em Flex when those become available, but you’re gonna have a harder time finding someone willing to do the scapel procedure on your butt I’d imagine.

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I was thinking a giant coil around your chair area might be possible to make but not too read chips, just to detect your body… or maybe a giant capacitive sensor would be needed.

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Well quite, but I don’t want any Tom, Dick and Harry to unlock my computer just by sitting on my chair. The identification part of it is needed also.

I think the idea @amal had was it would lock when you got out and bring up the login screen when you sit. No authentication. You could also do that with a laser trip-wire and probably be able to buy something off the shelf.

Edit: something like this:

Not super grindery maybe, but still cool.

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Yep. But the chip-in-buttcheek idea identifies me as I sit down. If I still have to stick my hand on the reader, there’s no added functionality really, so I’d go with the simple switch on the chair’s post.

I used to have a similar setup. I use a standing desk however so a butt implant never occurred to me. I actually tried a few ideas out over the years maybe some of them will inspire you:

Webcam + Face recognition

I made some code that checked for my face in the webcam feed every second or so and if it was missing for more than a small period it would auto lock. I only coded it to lock as a photo of me would have likely also worked.

False positives for when I turned around to talk to someone

Bluetooth + Phone or BT Tracker

Windows (I have not tested, I use linux)

Linux (I think this is what I used, it’s been a while)

It took a bit of effort to dial in the range without getting any false positives

GateKeeper Device

This little thing is what actually inspired my above attempts:
I never got one myself because I was a poor student and DIY was more fun

In the end I just ended up teaching myself to always press the lock button and my interest in doing it automatically waned.

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So here’s an idea after searching, you could use an Arduino kit. I found this tutorial for using two HF readers on one Arduino, and honestly I don’t see why you couldn’t scale it up if you do your homework. Then you could just insert these flat readers into the seat cushion in a few different spots and put a flex Nt in each check.

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That is a good point you could go for extra antennas. Not sure you would want to sit on a bunch of those pre-made PCBs. Multiplexing a bunch of wire or flexible PCB antenna would probably be more efficient and comfortable. Take a look at this if you want an example :slight_smile:

I have one too - a sophisticated electric thing that you can raise up and down. I hate it. I don’t know how people can work standing up. But that’s just me.

My interest in doing stupid things never wanes, even if ultimately the usefulness is debatable. Here I’m talking about creating the ultimate in PC logon laziness :slight_smile:

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Good find. Thanks!

There’s also this guy:

Apparently it has a 18" read range with a standard RFID card. Let’s say I get half of it with an implant, it could easily read my butt from under the seat. Or my hand from under the desk possibly, for that matter: as it happens, I already have an EM4305 chip in my right hand.

I think I’ll get me one of them boxes and try it out with my hand: If it works from under the desk, problem solved. If I feel it could work well enough from under the seat with less implant movement range than with my hand, then I’ll order another EM chip and I’ll make the embarrassing call to my piercer :slight_smile:


Same, but it shifts :sweat_smile: after that I invested far far too much effort in not having to touch my light switches ever again. Now I think of it, I may have given up on my PC unlock but my front door unlocked if I walk up my garden path :smiley:

I don’t like standing up by itself but I have this Ballance board thing (essentially a snowboard with a dome on the underside) that’s quite fun to twist and move around on.

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hah check this… ultrasonic presence sensor;

Yeah I saw this guy. But it’s not an improvement, as I would still have to present my hand to the reader under my desk to unlock the PC. I can achieve the same thing with a webcam and a motion detector. Also, it has a timeout to make sure the user is truly away - meaning it’s no better than… well, just the regular inactivity timeout in the Windows screensaver.

I’ve done some reading on what could be achievable on the 125kHz, 134kHz and 13.56Mhz bands, and I’ve come to the conclusion that you’re correct. UHF seems to be the way to go. Not sure how well an implanted UHF transponder would work though, with skin on top and flesh underneath potentially attenuating the signal. That would be something interesting to research.

As far as I can tell, there is no such thing as an implantable UHF transponder on the market. One would have to take apart a small animal tag (like a bird ring or something) and coat it in bio-compatible goop to turn it into something implantable. @amal: still looking for a coronavirus project? How would you feel about being the first person ever to create a ISO-18000-6 implant? :slight_smile:

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Not super awesome but normal 10 foot+ range probably not necessary here.

Actually I’m pretty sure a rival made one before… pretty big… in glass… terrible performance… probably could make some commercial UHF tag into a flex pretty easy and would have good-ish range. Find a uhf tag you wouldn’t mind implanting and add 2mm to length and width dimensions… we’ll talk.


That sounded interesting so I went and read some papers and thought I would share the relevant ones (You need to sign up for a IEEE account to access this one)

I do not know if you could get hold of any but these do exist UHF implants there tiny, so as I understand it the range is limited to about 5cm. It seems they use a Monza R6P chip (datasheet) hooked up to a dipole folded copper antenna printed onto kapton.
Butt (pun intended :laughing:) if @amal is interested in working on one you would likely get better range from a flex form factor or scaling it up to the usual X form factor that DF uses.

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