Small electromagnetic lock with additional safety mechanisms

Ideas and instructions how you can make your own bondage toys.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Small electromagnetic lock with additional safety mechanisms

Post by tina »

So I have been doing self-bondage for quite some time, but have never really done inescapable bondage because I'm always scared of earthquake/fire/etc. I understand some people decide to take that risk, and that we all take risks in everything we do, but wouldn't it be great if we can eliminate/minimize those risks?

My idea is based on what I saw on Haus de Sade (http://hausdesade.com), which sadly seems to have closed. In many of their scenarios they use small round electromagnets as handcuffs (http://hausdesade.com/images/Third-update-window.gif http://hausdesade.com/images/Haus-Mate-007.gif), as well as for locking body parts to... things. They use wires, but I'm thinking battery-powered. Now I also don't know how much force they can hold. It's a bondage site, so the models probably weren't struggling for real (someone correct me if I'm wrong?).

I know there have been some attempts at using door electromagnets for the same purpose, but they are quite huge, require a lot of power (making battery-power impractical), and obviously way overkill. Can you really pull 2000N (~200kg/400lb) with arms behind your back? Well, maybe you can, but I sure can't :D.

I'm thinking about something like this - http://www.ebay.ca/itm/55-LB-25kg-Elect ... 1#shpCntId

As you can see it's quite small (40mm diameter by 20mm height), and is rated for 660mA @ 12V, which is definitely possible to power with reasonably sized lithium ion/polymer batteries. Haven't done real calculations yet, but 2 AA-sized lithium cells (18650 for the technically inclined) should be able to power it for about 2 hours at maximum strength(*).

I can 3D print a case that holds the electromagnet, a small PCB with a wireless receiver module and USB port for charging (charging a handcuff with your phone charger? yeap!), and the li-ion cells. It would receive lock/unlock commands from a transmitter device connected to a PC through USB. It's possible to just use bluetooth too with a standard bluetooth dongle, but having a custom transmitter allows me to implement a few safety features, detailed below.

Safety -
- If the PC program crashes or the transmitter breaks or the handcuff gets out of range, the handcuff will stop receiving a periodic keep-alive signal, and the handcuff can be programmed to unlock if it doesn't hear from the transmitter in say 1 second.
- If the handcuff firmware crashes, it will unlock when battery runs out.
- The transmitter can have an accelerometer to detect movement (earthquake), and send an unlock signal. That means the transmitter must be unreachable and on an unshakeable surface when tied up.
- Detecting fire/smoke is very hard, but fortunately, someone did all the hardwork already - in smoke detectors. All the transmitter needs to have is a microphone to unlock the handcuff if there is very loud noise. I guess that means the subject must be gagged? Having heard how loud my fire alarm is, I'm pretty sure I'd be able to find a threshold that cannot be activated by screaming. There is also difference in frequency - fire alarms are typically very high pitched, and adult males are incapable of going that high (if you are an underage male, you shouldn't be here). Girls I don't know, someone wants to volunteer? :P
- Any more suggestions?

All the electronics design/programming is very easy, so I'm really only worried about holding force. If it requires a much bigger electromagnet this may not be feasible.

Cost of the whole thing would probably be on the order of $100 at small quantities (and I'm not going to mass produce this obviously). All designs will be published into public domain.

First step, ordering a few electromagnets to get a feel for how much force is required, unless someone has done this experiment already?

Also, I keep referring to it as handcuff, but it's really just a generic lock that can lock any 2 things together. Eg. it would also be possible to use it as a timed leash, and simply have handcuff keys out of reach. That would require much lower holding force, unless you want to kill yourself by strangulation.

Expect updates as this project goes on. I'm hoping to have it done in about 2 months, but PCB lead-time would probably be the longest.

This is much cleaner and reliable than ice, etc.

* For the even more technically inclined, one potential battery life optimization is to sense inductance by slightly fluctuating the voltage and looking at dI/dt, and apply more power only when the lock is being pulled (decrease in inductance)... it would very significantly extend battery life and is just pure evil. And no, I did not think I would ever use calculus on boundanna :rofl: .
Tenderfoot88
***
Posts: 458
Joined: 17 Sep 2013, 08:00
Location: British Columbia, Canada

Re: Small electromagnetic lock with additional safety mechan

Post by Tenderfoot88 »

tina wrote:adult males are incapable of going that high
I've never actually tested it, but my falsetto can get well into the ear-piercing shriek range that smoke alarms run at...for the couple of seconds it takes to disintegrate into a coughing fit.

The device, however, sounds pretty awesome. I think the main reason people are using door lock electromagnets is because they come with a sufficiently strong casing, and are useful in a wider range of circumstances (I might not to be able to pull 55 pounds apart behind my back, but I certainly could pull away in other positions. And I can't think of a position where legs wouldn't be able to pull 55 pounds...well, unless you're really small.

A lot of those door magnets, by the way, wind up using 12V power, which is easily doable with a couple of those big 6V batteries made for high power flashlights, or just using a car battery. Certainly not something you can mount on a handcuff itself, but you can run wires from a nearby battery case.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

That is a good point. 55 lb is definitely not enough for legs. I got both the 55lb and slightly bigger 80 lb versions to try out, so I guess we'll see how that goes.
lj
Moderator
Posts: 2255
Joined: 14 Oct 2008, 18:22
Location: East Anglia, UK

Re: Small electromagnetic lock with additional safety mechan

Post by lj »

a few techie points

door-hold electromagnets are available in very low-current versions - I bought one on EBay to try and found it was very secure running from a PP3 (nominal) 9volt battery - approx 200mAH capacity (often used for radios, smoke alarms etc, not sure if PP3 is an international type name) and as the electromagnet was running at about 80mA that makes for a safe back-up release. .This is well below 8 watts, more like 0.8 watts !

I wouldn't rely on a battery run-down to act as a firmware safety feature. Using a processor like most in the PIC range allows the use of the watchgdog timer feature included as standard. You could go a stage further and use a variety of chips or even just charge a capacitor on a FET gate, the FET being used to hold a normally open relay contact closed, using a pulsed output from the primary processor. The ideaof using a "must receive a signal every second" is an excellent watchdog in itself, something I have used in commercial designs using a radio link.

I'm not convinced by the inductance-sensing idea. The holding force drops away very quickly with separation and the difference in inductance between the hold state and the unrecoverable separation might be too narrow to be feasible. and no, I haven't done the maths !!!

A safety comment, I'd avoid LiPo batteries unless they are "hard-cell" types, the soft pouch type often used for RC model aircraft can catch fire if damaged (I've seen it happen) and the fire cannot be put out as the combustion reaction is self-supporting.
be a switch, double the fun :-)
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

door-hold electromagnets are available in very low-current versions - I bought one on EBay to try and found it was very secure running from a PP3 (nominal) 9volt battery - approx 200mAH capacity (often used for radios, smoke alarms etc, not sure if PP3 is an international type name) and as the electromagnet was running at about 80mA that makes for a safe back-up release. .This is well below 8 watts, more like 0.8 watts !
That is great to know! I guess it really depends on the application. When I get my magnets I'll try to run with lower voltages/duty cycles and see what the minimum is from different positions.
I wouldn't rely on a battery run-down to act as a firmware safety feature. Using a processor like most in the PIC range allows the use of the watchgdog timer feature included as standard. You could go a stage further and use a variety of chips or even just charge a capacitor on a FET gate, the FET being used to hold a normally open relay contact closed, using a pulsed output from the primary processor. The ideaof using a "must receive a signal every second" is an excellent watchdog in itself, something I have used in commercial designs using a radio link.
Using a WDT is a great idea. Will probably do that. Thanks. Hardware-based solution is pretty cool, too.
I'm not convinced by the inductance-sensing idea. The holding force drops away very quickly with separation and the difference in inductance between the hold state and the unrecoverable separation might be too narrow to be feasible. and no, I haven't done the maths !!!
That is quite likely. It was just a random idea that came up. I also didn't do the math!
A safety comment, I'd avoid LiPo batteries unless they are "hard-cell" types, the soft pouch type often used for RC model aircraft can catch fire if damaged (I've seen it happen) and the fire cannot be put out as the combustion reaction is self-supporting.
Yeah I was thinking 18650 cells. Though if the case is sufficiently thick (made out of ABS), battery damage is unlikely , too. Looks like some drop testing is in order.
User avatar
Riddle
****
Posts: 1136
Joined: 24 Sep 2008, 08:37
Location: Oregon, USA
Contact:

Re: Small electromagnetic lock with additional safety mechan

Post by Riddle »

Your idea looks to be a good plan. I like the multiple safeties and the inclusion of sensors. For a better detection of smoke, you could try directly sensing smoke with a smoke detector. Some new models may be wired together so all will go off in unison. That signal could be used to signal for an immediate release. I will be upgrading my home smoke detectors to them soon so I can use them to alert other systems. My self-bondage timer and alarm clock could use the information.

For battery power, I use 12V AGM (lead acid alarm battery) sized appropriately for the time. They can be purchased in sizes from 1 to 100 amp hours for reasonable prices.

Good luck with your project.
Resident timer maker. :hi:
Let’s make timers together!
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

There are smoke detector modules available, too, but the problem is then the transmitter would have to be placed strategically, which means much easier to get wrong.

I would also prefer not having to wire things into other things in the house for portability. Also, some people, like me who live in an apartment are not allowed to change the smoke/fire detector. They are all centrally managed and maintained for everyone's safety.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

Does anyone know where to find a metallic (either iron or carbon steel) disc/plate?

About 1.5 inch diameter, 0.5 inch height.

I can't seem to find them anywhere. Probably looking at wrong places or using wrong search terms. I can get them waterjetted, but that's expensive.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

First iteration of the schematic done. Gonna wait till the electromagnets come in to finalize a few components (FETs, etc).

Feedback welcome!


http://picpaste.com/eml.png

PS. It looks like the forum's attachment upload is broken? I finally found a format it would accept, and it just gave me a blank page after upload.
Last edited by tina on 15 Jan 2014, 10:20, edited 2 times in total.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

I've decided to move this project to a blog since I will probably end up with a few projects, and it's easier to centrally track them. Also nice to be able to attach files, etc.

http://tinatiedup.wordpress.com/

Feel free to check out my other project ideas on there, too!

I'll still update this thread if there is significant development on this project.

Thanks
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

Update! The electromagnets arrived, and I played with them with my bench power supply http://tinatiedup.wordpress.com/2014/01 ... etic-lock/

TL;DR: Very impressed. One of them at 0.6W (that's about 2.5 hours on a single AAA battery!) is powerful enough that I can't pull it apart even with my back muscles.
tina
*
Posts: 28
Joined: 06 Jan 2014, 11:16

Re: Small electromagnetic lock with additional safety mechan

Post by tina »

And the PCB designs have been done.

Base station:
Schematics - https://s3-us-west-2.amazonaws.com/tnf/base_sch.pdf
PCB - http://oshpark.com/shared_projects/3jK4S2tj

Lock:
Schematics - https://s3-us-west-2.amazonaws.com/tnf/eml_sch.pdf
PCB - http://oshpark.com/shared_projects/J3kgAyDd

Should be here in about 3 weeks! Time to start on the mechanical design.
User avatar
nitro
Retired Moderator
Posts: 463
Joined: 18 Aug 2008, 22:04
Location: North East US

Re: Small electromagnetic lock with additional safety mechan

Post by nitro »

Another safety option would be to add an analog watchdog circuit to the lock.
Basically a series diode and cap with an RC and a FET designed so that a pulse every n sec will keep the FET on. if the pulses stop (stuck high or low) or fall outside the design parameters the circuit will turn off.
[img]
watchdog.png
[/img]
Tenderfoot88
***
Posts: 458
Joined: 17 Sep 2013, 08:00
Location: British Columbia, Canada

Re: Small electromagnetic lock with additional safety mechan

Post by Tenderfoot88 »

What did you have in mind as an input?

Heart rate is too naturally variable, and by the time you hit something that's guaranteed to be alarming (dropping below 40ish or 55ish depending on whether or not you meditate), that's often because you're swiftly headed towards zero and don't have any possibility of self-powered escape due to weakness.

O2 sats would probably do a little better - I'm not sure where the threshold of actual danger is, but I know they get concerned if you're not at 100% without heavy exercise or holding your breath. But then, that's getting into the rather intense side of medical equipment - you rarely see O2 sats monitoring equipment outside a hospital, especially when it's meant for continuous monitoring.
User avatar
Keyless
***
Posts: 344
Joined: 22 Dec 2013, 12:33

Re: Small electromagnetic lock with additional safety mechan

Post by Keyless »

tina wrote:I've decided to move this project to a blog since I will probably end up with a few projects, and it's easier to centrally track them. Also nice to be able to attach files, etc.

http://tinatiedup.wordpress.com/

Feel free to check out my other project ideas on there, too!
and
tina wrote:
For sensors, something like a gyroscope with an accelerometer would allow you to detect orientation, which enables projects like something that forces you to run around in a circle in some kind of predicament some number of times before releasing you.

A GPS sensor can be used to force you to jog at a certain speed, or it will "encourage" you in some way (an e-stim spank?)
I love the idea of the GPS marathon trainer on your blog. I tried using GPS myself for making a commitment to exercise. This is part of the post.
My doctor knows I like cycling and has told me to go on cycle trips more often so as to get more exercise. I spent a long time trying to devise something to allow me to plan a trip and face some sort of penalty if I chicken out. After a lot of thought and rejected ideas, I decided that GPS was the only real solution. I have an Arduino and the Adafruit GPS logger, together with a program on my laptop. I can't say it was easy but, at last, I have a program that allows me to enter a planned trip with up to 6 waypoints, dates between which the trip must be made, and a final date and time after which I can be released without making the trip. When I lock it, the system will encrypt a photograph. Once that has been done, the photograph can only be decrypted by making the trip or waiting until after the final date. I believe I have made it cheat proof.
It works well and it really has helped me to improve my fitness. Normally I have my cycle suit locked on and I have to complete the trip to generate a GPS log that will allow me to decrypt the photograph to get the combination to the safe containing the key to the suit. It does not provide instant punishment as your idea does. I also note that you mention accelerometers. I wonder if it would be possible to incorporate all these ideas in a high tech chastity belt. It could be made of a light weight material, perhaps fibreglass. It could be designed to be removed easily in an emergency and should be less likely to cause injury in an accident than a heavy steel belt. It would have various electronics moulded into it. The most basic belt would have tamper proofing wires running through it. Cutting or opening it without authority would destroy some sort of encryption key or code, with highly undesirable (maybe expensive) consequences. There could be all sorts of additional features. A processor, radio module for communication with a PC, an electrostim punishment unit and various sensors. GPS and an accelerometer would be obvious choices. It could also have extra memory for logging. The radio module could be configured to provide remote control punishment or an electronic leash, whereby the wearer gets punished if out of range of a transmitter. The belt could tell where the wearer was, the direction and speed of movement and perhaps sense walking and running. The wearer could commit, or be committed by a partner, to particular behaviour over a period of days or perhaps weeks. The wearer might be made to walk or cycle to work, be punished for lateness or made to carry out a jogging or cycling exercise routine. He could even be punished for being late to bed or not getting up on time! With a temperature and pressure sensor it might be possible to tell when the wearer is in water, so enforced swimming is a possibility. It would also be possible to arrange for it to be controlled over the web, via the computer.

There is no way I will ever have time to try such a thing, but I don't think it is wholly impossible. The big problem is that you would have to do a lot of explaining before attempting to get through a security scanner.
Post Reply