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They are relatively rare in fiction but there are a few interesting examples, in fact I even remember reading about one in real life.

I would like mine to be small and light (say less than 0.9 kg and less than 2 cm in diameter). Death must be assured if the conditions are not met. So, can we build it with current tech?

EXTRA POINTS: it would be great if there was room for extra stuffs, say a microphone, GPS or other things...

Recharge is also an issue, can they last indefinitely if powered by body motion like clocks?

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    $\begingroup$ @JohnRennie Do not answer in the comments! Post it as an answer! $\endgroup$ – PipperChip Jan 5 '17 at 12:57
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    $\begingroup$ The answer to any such question is "yes we can do it, no we can't do it bug-free." Do you really want your boom collars exploding because a GPS drift bug placed the wearer in China for a split second? $\endgroup$ – SPavel Jan 5 '17 at 15:43
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    $\begingroup$ @PipperChip - "LMGTFY" is frowned upon as an answer (end @). And IMO, so are OP's questions that don't google their own titles. reality-check is also inappropriate, as these devices moved beyond the "concept" phase over a decade ago. Do explosive collars exist is off-topic, and Help me build one most certainly should be. - AFT knocking on your door in 3, 2... $\endgroup$ – Mazura Jan 5 '17 at 22:44
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    $\begingroup$ There was a movie with Rutger Hauer that features such collars, rigged to explode if removed or if a matched pair of them get more than X feet away from each other. Every time I forget my phone and my Bluetooth torque vibrates, I think of that. The only thing left to implement on such a collar would be the small explosive, and those have been around for decades. Everything else can be done with commodity mobile tech. $\endgroup$ – T.E.D. Jan 5 '17 at 23:25
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    $\begingroup$ @PipperChip but then you’d flag the link-only answer! If he's not inclined to develop it into an answer, leaving the link for others is just fine. $\endgroup$ – JDługosz Jan 6 '17 at 8:43
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Quite realistic.

Any low level smartphone is more than capable of being used as the control terminal. If you want it rugged and with good battery life, let's say that it weights 300 gr (with all the extras you wanted).

This leaves 600 gr. for explosives and the strappings to the neck... more than enough to kill not only the bearer but a whole bunch of people around.

As for the recharge, there are two options:

  • If you want to use it to keep your target in a secured area (so they cannot escape a prison), you can use some of the new wireless recharging technologies that are just becoming available.

  • If you want to use it to control remotely someone, just nicely explain to him that if the battery becomes lower than 10% an auditive signal will be noticed, and when the battery gets to 5% the device will explode. This is what in psychology is called "providing an incentive" for someone to do some task.

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    $\begingroup$ You don't even need a smart phone. A dumb phone, or even a dev board with SIM slot is enough. A smart phone can be rooted and the battery level faked. A dev board with a custom (enough) OS, not so much. $\endgroup$ – jaxad0127 Jan 5 '17 at 17:05
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    $\begingroup$ We are already getting close regarding the explosion on discharge. Li-Ion power cells don't explode on discharge, but they explode when you try to charge them again. $\endgroup$ – John Dvorak Jan 5 '17 at 17:08
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    $\begingroup$ Better yet, just strap a Note 7 to the target. Control unit and explosive, all in one! $\endgroup$ – user14624 Jan 6 '17 at 12:01
  • $\begingroup$ Comparing to a smart phone, I believe it would be easy to make the batter last 1-2 weeks. $\endgroup$ – Sulthan Jan 6 '17 at 12:34
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    $\begingroup$ How about a pager? They're still supported by the networks, easy to hack into, and (used to) get weeks of battery life off an AA cell. With the limits in the Q you could probably squeeze in enough batteries for a year $\endgroup$ – Chris H Jan 6 '17 at 14:20
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The case of Brian Douglas Wells is exactly what you are looking for.

Brian Douglas Wells (November 15, 1956 – August 28, 2003) was an American pizza delivery man who was killed by a remotely controlled bomb fastened to his neck, under coercion from the maker of the bomb. After he was apprehended by the police for robbing a bank, the bomb was detonated. The bizarre affair was subject to much attention in the mass media

In other words - so realistic that they actually exist.

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    $\begingroup$ yup! That's the case I remembered, however from the wikipedia article I recall it was quite a bit thing $\endgroup$ – SilverCookies Jan 5 '17 at 15:19
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    $\begingroup$ Why is this not the accepted answer? $\endgroup$ – Mawg Jan 6 '17 at 15:31
  • $\begingroup$ That collar was actually fairly large. It was basically a bomb chained to his neck. Also, it had a timer not a radio. $\endgroup$ – Malvolio Jan 7 '17 at 1:40
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You are talking about det cord

Det cord is a cable that goes boom. If you wrapped this around someone's neck....well I don't know what would happen but it would be messy.

You can rig it to go off with a cell phone detonator. This is the basic principle behind IEDs. I can't send you a link, but if you don't mind being on a terrorism watch list you can go investigate on the interwebs.

So yes, you can make an explosive collar with current technology.

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  • $\begingroup$ Thanks for the idea, I don't plan on actually doing it nor do I want to be on any watchlist :P $\endgroup$ – SilverCookies Jan 5 '17 at 15:17
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    $\begingroup$ @SilverCookies Improvised Explosive Device. $\endgroup$ – kingledion Jan 5 '17 at 15:17
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    $\begingroup$ To give some idea of the power of det cord, it can be used to fell trees. $\endgroup$ – Matt Bowyer Jan 5 '17 at 15:24
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    $\begingroup$ I suspect that it would be rather messier... $\endgroup$ – Matt Bowyer Jan 5 '17 at 15:31
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    $\begingroup$ I always thought IEDs should have been called Macgyver Devices. $\endgroup$ – NZKshatriya Jan 6 '17 at 2:18
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Engineering Considerations

The other answers do a good job of summarizing that yes, you can absolutely make an explosive collar with current technology. There are some edge case factors that might be relevant to consider.

Collateral damage:

If you have someone with the trigger who has some kind of moral issue with excessive collateral damage, wants to ensure that they definitely have the CORRECT target before the device goes boom, etc, then you might want to think about the use of an explosive cellphone on a terrorist leader by the Israeli Mossad in the early 2000s. The cell phone was used because the IDF could then confirm that the correct person was holding the phone. According to people who had knowledge of the operation, the main engineering challenge was NOT to make a fully functional cellphone with enough room in the case to house enough plastic explosive to blow someone's head off, but to keep the blast directed enough and small enough to ensure that there would not be collateral damage to other persons nearby. Israel didn't want to accidentally kill a waiter or something if the target was at a restaurant. An explosive collar would have a similar engineering consideration. It would only need a pretty weak explosion to take off someone's head if your charge were literally wrapped around their neck. In the case of the phone, I believe the back of the phone housing was reinforced with steel and shaped in a way to direct the blast out through the face of the phone. A collar would have a similar consideration, so the outside of the collar might be a steel housing that forces the blast from the charge inward.

Det Cord

Det cord was mentioned, but in my opinion, it would be overkill. We used to use that stuff to blow open steel doors... in my non-techie life... You probably would have a risk of collateral damage with normal sized det cord. Probably something about a quarter the diameter would be fine.

Weather Proofing

Waterproofing/weatherproofing is not a major problem. The explosives themselves can be waterproof. The major issue is to waterproof the battery container and make sure it doesn't accidentally short under certain environmental conditions. This is somewhat expensive (compared to cheap consumer electronics) but is a challenge that has absolutely been solved elsewhere.

Recharging

There are ways to harness the natural electrical current of the human body. If you wanted to get fancy, you could possibly harness this to recharge a very small battery in the collar. This might not withstand rugged use/real life/weather etc however.

Trigger

As far as detonation, the obvious choice is a cell-phone type trigger. This is used by lots of terrorists all over the world. It may be a bit risky to go this route though, since there are ways to send out a lot of signals that will preemptively detonate such devices in a given area. A better solution might be a combination of a radio signal and software that verifies the identity of the signal sender via a pre-selected code. That way some stray signal doesn't accidentally blow off your guy's head before it is time.

Answer: Totally Doable

All in all, if you invest some money and have some good engineering skills available, you could absolutely make a collar that would last a very long time, survive normal life, and only go boom when you want it to.

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    $\begingroup$ LOL - I get robo calls even on my cellphone now even through my number is the the US do not call list. So it would seem that some sort code for the phone call would be needed. $\endgroup$ – MaxW Jan 5 '17 at 17:57
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    $\begingroup$ A small amount of Detcord at the base of the skull would probably be enough to cause an internal decapitation without the potential collateral damage. You only need the victim to die. Think more of a lethal fizz, more than a boom. Although a boom might act as a deterrent for the other prisoners. $\endgroup$ – Paul TIKI Jan 5 '17 at 19:37
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    $\begingroup$ I like your point about collateral damage. If you are really worry about collateral, I propose an alternative. Instead of exploding, on activation the collar contracts to the circunference of a doughnut. That should keep the people around (reasonably) safe; the collar-wearer... not so much. $\endgroup$ – xDaizu May 9 '17 at 13:36
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Absolutely possible

1 kg of plastic explosive

^ That is 1 kg of plastic explosive going off, which i'm sure that even 0.5 kg of that, would be more than enough for a very messy and sudden death.

As far as transmitter/gps/microphone equipment goes, we have smartphones that weight only 100g:

lightest phone

So yap, these are not only possible, you wouldn't even need to be tony stark to build one.

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    $\begingroup$ If you have plastic explosive next to someone's spinal cord, 1kg is definitely overkill. $\endgroup$ – matsjoyce Jan 5 '17 at 20:25
  • $\begingroup$ A kilogram? He wants to kill the guy, not obliterate his body from the planet. A half-kilogram will destroy a car or small truck. $\endgroup$ – Malvolio Jan 7 '17 at 1:58
  • $\begingroup$ @Malvolio lol, I'm simply outlining what his "Michael Bay" budget is with his weight limitation. $\endgroup$ – padleyj Jan 9 '17 at 13:33
  • $\begingroup$ Wow...just...Wow. That is a big boom. $\endgroup$ – Paul TIKI Nov 18 '17 at 14:03
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You can totally create a nuclear device with any size. It only need an element that sustains a nuclear fission chain reaction, for example Uranium 235:

enter image description here

Or plutonium 239:

enter image description here

In these examples, you just need a neutron to start the reaction.

You can also calculate the energy produced by getting the difference in mass of the products (in case on the Uranium one is Krypton 91 (which decays to Krypton 89), Barium 142 and 3 neutrons):

enter image description here

In contrast with conventional explosives, 500g of Uranium is about the same as 1500 ton of coal.

About the critical mass

One can argue that these reactions can't occur if the elements are not at the minimum wight/size requirement for sustain a chain reaction, called critical mass. However, a workaround is to wrap the element with a layer of graphite. It's called "Neutron Reflection". Read more here

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    $\begingroup$ Neat. The question was not, however, how to obliterate completely everything within a radius of a mile or two... $\endgroup$ – chx Jan 5 '17 at 22:25
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    $\begingroup$ Wrapping it in a neutron reflector lowers the critical mass. It doesn't eliminate it. Besides, the explosive assembly of the critical mass will kill the target before you look at the nuclear effects. $\endgroup$ – Loren Pechtel Jan 5 '17 at 23:31
  • $\begingroup$ not to mention the radioactivity of the fissile materials if not shielded well....... $\endgroup$ – NZKshatriya Jan 6 '17 at 2:20
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    $\begingroup$ Overkill doesn't even begin to describe this. This is a M-M-M-MEGA KILL kill kill kill.... $\endgroup$ – Dan Ross Jan 6 '17 at 3:33
  • $\begingroup$ Those atoms in the first picture look delicious $\endgroup$ – user1975 Jan 6 '17 at 3:56

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