Inescapable electromagnetic wrist lock

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Inescapable electromagnetic wrist lock

Postby Onwrikbaar » 06 Oct 2018, 13:52

This simple design has proved perfect for automated as well as remotely controlled captivity. It takes just a few hours to build, from readily available components together costing less than €40 (including a power adapter), using only common DIY tools.

Since there is no key or combination lock involved, one can use this lock while wearing a blindfold or hood.

IMG_1150.jpeg
The magnet is a standard 12V DC door holding type.
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Re: Inescapable electromagnetic wrist lock

Postby CD Tammy » 10 Oct 2018, 04:18

I have one of these powerful door magnets. Most of my restraints with them involve my handcuffs attached to a piece of metal that is stuck to the magnet.
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Re: Inescapable electromagnetic wrist lock

Postby Onwrikbaar » 01 Nov 2018, 11:51

One of the great things about these magnets is that they use so little power that they can be built into wearable, battery-powered restraints (which could have a timer on board or even be controlled through Bluetooth or WiFi).
For instance, the magnet type as pictured in my photo (which has 60 kg holding force) uses only between 100 and 150 mA @12V, so less than 2 Watts. That means that a battery holder containing three freshly-charged 2600 mAh 18650 type LiPo batteries in series will power it for at least 17 hours.
Last edited by Onwrikbaar on 07 Nov 2018, 14:16, edited 1 time in total.
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Re: Inescapable electromagnetic wrist lock

Postby red_bondage » 07 Nov 2018, 13:22

Hey out threre,
°
I must admit this idea is quite intriguing to me. This is the third time I am looking at this picture, but I am still not sure if I understand what is going on here.


On the right side of the wrist is just a hinge, to open and close the top part of the cuff. Right?

The black stripe in the middle, on top of your wrist seems to be "just" tape to prevent scratches on your arm?

Left side of the wrist there is the wooden dowl. I guess it is there to prohibit left / right movement of the looking metal bar.

On the very left there seems to be the magnet itself (the black box?) On top of that a block of iron, screwed with the one large screw to the looking metal bar. I guess this is needed because the looking bar across the top is aluminum?

To get out, you need to rotate your wrist 90°, and that is only possible if the metal bar is moved out of the way.

Is this approx right?
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Re: Inescapable electromagnetic wrist lock

Postby Onwrikbaar » 07 Nov 2018, 14:13

red_bondage wrote:On the right side of the wrist is just a hinge, to open and close the top part of the cuff. Right?

The black stripe in the middle, on top of your wrist seems to be "just" tape to prevent scratches on your arm?

Left side of the wrist there is the wooden dowl. I guess it is there to prohibit left / right movement of the looking metal bar.

On the very left there seems to be the magnet itself (the black box?) On top of that a block of iron, screwed with the one large screw to the looking metal bar. I guess this is needed because the looking bar across the top is aluminum?

To get out, you need to rotate your wrist 90°, and that is only possible if the metal bar is moved out of the way.

Is this approx right?


Hello red_bondage,

Close enough :D
The black sleeve to protect the wrist is actually a piece of bicycle inner tube, simply slid over the aluminium top bar. With everything else you are spot on.

The ferromagnetic plate comes with the magnet. These magnets can be bought from several places, including AliExpress (https://nl.aliexpress.com/item/Electronic-Lock-Electric-Magnetic-Door-60kg-100lbs-Holding-Force-Electromagnetic-Mini-M60/32616532185.html) and Amazon (https://www.amazon.com/AGPtek-Electric-Magnetic-Electromagnet-Fail-Safe/dp/B00JERC00S/).
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Re: Inescapable electromagnetic wrist lock

Postby red_bondage » 13 Nov 2018, 06:55

Thanks for the feedback :)

But I have one more question:

It is possible to close this cuff with just the "hand - to - be - locked"?
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Re: Inescapable electromagnetic wrist lock

Postby Onwrikbaar » 13 Nov 2018, 08:40

red_bondage wrote:It is possible to close this cuff with just the "hand - to - be - locked"?

Oh yes :-) It's easiest when the magnet isn't powered yet: you close the cuff, then lift the top bar a little with the hand you want to put in, insert your wrist, and the top bar lowers itself when you rotate your wrist in place. You don't even have to be able to see what you're doing - I use it all the time to capture my last free hand while wearing a closed hood.

I designed a little driver circuit for it that can detect when the cuff gets opened and closed. This way it doesn't matter how long my preparations take; the automated scenario doesn't start until I close the cuff.
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Re: Inescapable electromagnetic wrist lock

Postby red_bondage » 13 Nov 2018, 10:46

Thanks again :)

I will think about building one for myself.

Onwrikbaar wrote:I designed a little driver circuit for it that can detect when the cuff gets opened and closed. This way it doesn't matter how long my preparations take; the automated scenario doesn't start until I close the cuff.



Uhh... now that is quite an interesting idea. One more guess *smile* you are measuring voltage spikes?
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Re: Inescapable electromagnetic wrist lock

Postby Onwrikbaar » 13 Nov 2018, 11:49

red_bondage wrote:Thanks again :)

I will think about building one for myself.

You're welcome. If you happen to have a Fetlife.com account, you can view the parts list and detailed build instructions here: https://fetlife.com/groups/80/group_posts/12204734

Onwrikbaar wrote:I designed a little driver circuit for it that can detect when the cuff gets opened and closed. This way it doesn't matter how long my preparations take; the automated scenario doesn't start until I close the cuff.


Uhh... now that is quite an interesting idea. One more guess *smile* you are measuring voltage spikes?


You have obviously given this some thought :idea:
Watching for voltage spikes would indeed work to detect opening and closing. However, it would not be able to determine whether the cuff is open or closed statically, like on startup of the scenario program. Luckily, there's another way (apart from using a separate sensor of course, which I wanted to avoid): the magnet's coil's inductance is significantly larger with the metal plate on the armature (cuff closed) than it is with the plate removed (cuff open). When switching on the magnet, this difference in inductance causes the current through the coil to rise slower with the plate on it than without it. My Raspberry Pi program measures this time difference by means of the driver circuit.

The nice thing about using the physical properties of the magnet is that you can experiment with various detection methods without having to change or add anything to the cuff's construction.
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