Why would clipping a wire cause a bomb to explode?

Question

There are a lot of movies and video games that depict defusing bombs, most of which boils down to picking the right color wire. Something like this: enter image description here

Now, that's a part I don't understand.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

I'm not very familiar with circuitry and electrical engineering, but I assume this would be the place to ask it. I'm confused as to how clipping a wire would cause a bomb to explode. I assumed that if you clipped a wire, there'd be no connection from the switch to the bomb, so it wouldn't explode, but apparently, in a lot of movies as well, clipping the wrong wire leads to dangerous things.

schematic

^{simulate this circuit}

Is this a realistic scenario? I really don't understand why clipping a wire would cause a bomb to explode. Rather, shouldn't it "defuse" the bomb?

[Wire Dilemma](http://tvtropes.org/pmwiki/pmwiki.php/Main/WireDilemma) - Warning: TV Tropes link — Ben Miller - Remember Monica, Sep 17 '14 at 07:50
They construct the bomb so that the timer is the only thing stopping the bomb from exploding.exploding. — user253751, Sep 17 '14 at 10:45
Bomb makers still use wires? I thought that TI, ST, etc. already had integrated bomb detonation chips, with everything you need. — Zuu, Sep 17 '14 at 15:25
I feel like just asking this question has gotten all of us on to the FBI's watch list ;_; — yuritsuki, Sep 17 '14 at 15:26
@Mew cut the Vcc of the not gate, not the signal! Also; take a normally open solenoid(by spring), and add a scratch paper or similar. Then use the power cable to keep the solenoid closed and arm your bomb. When power stop keeping the solenoid closed... boom. But then just jam the solenoid. the problem is to retro-engineering the bomb before it explode. — Lesto, Sep 18 '14 at 12:08
It would be simple to design a mechanical detonator which was held back by an electromagnet. During World War II, the British used some rather large electromagnets to temporarily disable mechanical clockwork detonators on unexploded German bombs. As long as the electromagnet was powered, the bomb's clockwork mechanism couldn't run, but if power was removed, the mechanism's countdown would resume. Of course, the fact that the British electromagnets were externally applied required them to be much larger than would have been necessary if they could have been safely applied internally. — supercat, Sep 18 '14 at 16:12
If it explodes at all, it won't be more powerful than your [Plot Armor](http://tvtropes.org/pmwiki/pmwiki.php/Main/PlotArmor). (TvTropes, if you dare.) — corsiKa, Sep 19 '14 at 15:17

score 63 · Answer 1 · answered Sep 17 '14 at 04:32

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I suspect that there is an ISO standard that bomb makers follow. As evidence of this is the fact that in about 50% of the movies, there is a statement like "remember to clip the red wire and NOT the blue" or some similar statement. SO I suspect that embedded in the ISO document is a standard that indicates the standards for booby-trapping and which wires cause the bomb to explode and which cause the bomb to fail.

answered Sep 17 '14 at 04:32

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+1 It's ISO-0NO-1981, more colloquially known as the "oh no" standard. ;) – DrFriedParts Sep 17 '14 at 05:05
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@DrFriedParts: I cannot identify to which degree this is a joke. Are there any publications anywhere explaining it? – sharptooth Sep 17 '14 at 09:15
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@sharptooth this is a fun question, so I thought I'd have fun. It's not uncommon around here that if someone asks a question that isn't "serious" that some people get all snarky and write very dismissive things. I thought I'd be silly to counter act that. – placeholder Sep 17 '14 at 14:31
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@DrFriedParts - AKA - Yellow then Orange = Kill Off - the short hand mnemonic is YOKO. – placeholder Sep 17 '14 at 14:32
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@placeholder best answer ever – blarg Sep 17 '14 at 21:18
I really think this works only as an - awesome - comment, but doesn't really fit the answering standards. But then, even the question is borderline – clabacchio Sep 18 '14 at 19:12
@clabacchio Well, on some sites, it would have been marked CW, because we all want some fun, and making the fun rep-less (rap-less?) is much more acceptable to many. – yo' Sep 21 '14 at 19:50

score 48 · Answer 2 · edited Sep 20 '14 at 07:50

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As you have identified, clipping the correct wire would stop a bomb exploding.

So, a bomb maker would ensure that there are many wires, so it isn't obvious which one to clip.

They would monitor whether a wire has been cut, and if it is, the bomb would explode. They are bomb makers, after all. They could also add more stuff to detect whether the bomb has been opened, etc.

Many people have lost their lives trying to defuse bombs, and the UK army disposal folks prefer to either completely destroy the detonation system, or just blow the whole thing up when one is found.

So I have always assumed it is somewhat realistic.

Of course, I don't believe anyone would put a cute little LCD displaying the countdown. Nor do I think they would use different colors for each wires, or maintain a consistent color code across a set of similar devices.

edited Sep 20 '14 at 07:50

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answered Sep 17 '14 at 04:11

gbulmer

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I'd say that carefully color coding wires would make sense to prevent errors while proper assembling the device and to make its emergency deactivation easier by the people who activated it earlier. Same as with any electrical circuit. – sharptooth Sep 17 '14 at 09:20
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Point is, out of a large number of bomb makers, only a few will actually be sophisticated enough to build one that couldn't be defused simply by ripping out the wires and/or detonator. But since you will never know if you are dealing with that one evil genius, better be safe and blow the thing up with a powerful stream of lots of water. So unless this is not an option because the bomb is taped to someones head, no one would be so insane as to start cutting wires. – PlasmaHH Sep 17 '14 at 10:20
@sharptooth - colour coding wires is *not the only way* to identify wires. For example all the Ethernet cables through buildings *are* the same colour. If a piece of electronics has many more wires than colours, it would clearly become error prone, which is the very last thing you'd want. A better solution would be to use a [cable identification scheme](http://uk.mouser.com/Wire-Cable/Wire-Protection-Management/Wire-Identification/_/N-apvo7). Then remove the markers from the wires when it no longer matters. I think I've seen a movie where they did something like that. – gbulmer Sep 17 '14 at 11:53
@gbulmer: You can use both. For example, you have ten different colors for those cables and once you see that you can run out of colors you throw in those neat labels. This makes identification much easier and less error prone - if you've seen two identical labels of interest on a green cable you know that you have to continue following only green cables and check labels only on those. Spotting as single misplaces marker gets easier too. – sharptooth Sep 17 '14 at 12:38
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@sharptooth - Sure you *could* use both. However, the question is about defusing bombs in movies vs reality. To increase jeopardy, and avoid being disarmed by following a colour chart, the wires *should* be harder to distinguish. You opined that a bomber would carefully colour code wires. I simply pointed out that colour coding wires is neither necessary nor sufficient. Further, IMHO a bomber, of the criminal master mind variety, would figure out that to "make its emergency deactivation easier" seriously reduces the difficulty of defusing it, which fails for bombs in movies and reality. – gbulmer Sep 17 '14 at 12:49
@sharptooth - Of course, if you have more experience, please share !-) – gbulmer Sep 17 '14 at 12:52
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@sharptooth a recent episode of Sherlock used the premise that of course the bomb makers are going to have an easy way to turn it off, in case they need to use it themselves. – Michael Sep 17 '14 at 15:07
@Michael - Yes, that *completely failed* as a dramatic plot-device, but was amusing as a joke. Though IMHO Sherlock doesn't stand up very well to rational examination at the best of times. It's an ironic 'sit-com', with some darkness, and works well as that. It is rather more 'tongue in cheek' than the old Mission Impossible TV shows. – gbulmer Sep 17 '14 at 15:14
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I don't think that the bomb maker needs to color code the wires to be able to defuse the bomb himself.I mean when I build something (I have never built a bomb,but I imagine it wouldn't be that different),I can remember the circuit diagram and where all the wires go for some time,so the bomb maker shoudl know which wire can deactivate the bomb.Of course,a better way to do it would be to make all the wires make the bomb explode and to defuse the bomb one must short two wires.Or use a 50 wire ribbon cable with the one true wire somewhere in the middle. Also, no LCD and no beeping. – Pentium100 Sep 18 '14 at 06:17
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Personally I'd include an LCD but make it inaccurate. For example it could have 30 seconds left after 00:00 just to give people that sense of relief before blowing them up. Of course, making it display 5318008 and having the detonator trigger on giggle detection would work too. – moopet Sep 18 '14 at 10:31
@moopet #15 http://www.eviloverlord.com/lists/overlord.html – Dan Is Fiddling By Firelight Sep 18 '14 at 11:05
I will disagree that cutting the right wire would always defuse the bomb. If you want to make it really hard on the EOD guys you have two detonators, each of which is normally part of the circuit that keeps the other one from detonating. Each circuit also has it's own power source. There's *NO* wire you can cut that won't set it off. – Loren Pechtel Sep 19 '14 at 21:28
I'd totally go with color coded wires. I already make enough mistakes soldering wrong wires together, that are color coded, on less dangerous (non-bomb projects). I'd also go with Sherlock, a switch would be quite useful in the testing stages, as it has proven to be when testing robots. – cubecubed Sep 20 '14 at 15:00

score 38 · Answer 3 · edited Sep 17 '14 at 16:42

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When I was in Afghanistan most bombs (IED's) followed a pattern for the area it was made in. A few bomb makers taught others to make them there way and they taught others and so on. Neatly colored wires were not common in the Pech. Most of the time they were wired so someone watching the bomb could detonate when a target got close. I have been on the receiving end of a few of these. No fancy display counting off time. Simply a enclosed container with a combustible hooked up to a battery wired to a cell phone. When the phone got a call, the electricity would flow through the wires creating a spark in the container with the combustible and expanding in a enclosed space forcing matter outwards—and boom. Now I'm a 100% disabled college student who can't lift a lot of weight, lost a lot of friends, and has trouble with steps.

edited Sep 17 '14 at 16:42

JYelton

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answered Sep 17 '14 at 15:26

Phatez

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What appears counter-intuitive [being polite here] to me about the US operating procedures is that they let the cell service operate. At least, that's how it seems. Sure, there are other ways of triggering an IED remotely (or even personally). But is it a good idea to let them use a cheap and convenient communication system. – Nick Alexeev Sep 18 '14 at 20:04
@NickAlexeev they can't just arbitrarily disable entire civilian communication networks. What do you think they are, syria/china? Also, a cell phone could easily be programmed to do the opposite. Wait for a cell jammer to kill the signal, triggering it, or trigger if a call doesn't come in after x minutes (dead man's switch) – Passerby Sep 19 '14 at 05:23
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@Passerby Neither Geneva, nor Hague convention says that the occupying force has to maintain cell phone service for the opposing guerrilla force. US forces have instituted a curfew for people. Might as well do something about cell phones. – Nick Alexeev Sep 19 '14 at 05:53
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Enter the THOR, man-portable IED Jammer. I think it jams GSM as well, however I don't know how much it's used. http://en.wikipedia.org/wiki/Thor_III – Rick Sep 19 '14 at 10:54
In the BBC documentary "The Bomb Squad" the ATO (Ammunition Technical Officer) was accompanied by an electronic warfare specialist with jamming equipment. Excellent documentary - wire seemed to be whatever the bomb maker had available. – MZB Sep 20 '14 at 23:25

score 30 · Accepted Answer · edited Sep 21 '14 at 15:57

There are plenty of ways of having an active open circuit. Below is just one simple example, using a single PNP transistor.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

The Blue wire carries the power to the Primer/Explosive/Bomb. Current through the PNP is blocked by holding the base high through a resistor and the Red wire. Cut the Red wire and the PNP transistor's base is pulled low, allowing current through and GAME OVER MAN, you're dead. Cut the Blue wire, you are safe.\

Now multiply that by multiple wires, multiple transistors, or relays, or multiple paths, microcontrollers, False wires, etc, and that's how you get a realistic hollywood scene, logically speaking.

Come on, you could add some redundancy there, maybe put a minor sense current through a pair of detonators so they can check each other.... — Nick T, Sep 20 '14 at 02:16

score 20 · Answer 5 · answered Sep 17 '14 at 04:11

The answer is hollywood logic.

Really, though. If you cut the power, it's dead. If you cut the detonator, it's dead. However, if you cut a wire that looks like the power wire, but is actually connected to some logic that detonates the bomb if it's cut, then it will go boom. If the bomb builder is clever with how the bomb is wired, then it will not be obvious which is the real power wire or the real detonator wire and which are the booby traps. I'm really not sure how realistic that scenario is, though. I don't think real-life bombs are ever defused the way you see in the movies. Which is probably why the bomb squad usually just clears the area and then blows them up.

Hmmm. Large cap + big pull-down resister on power supply, and power supply is connected to data pin on PIC controller. Cut power -> BOOM. Cut the PIC's power pin instead. — Joshua, Sep 17 '14 at 19:02
True. Using a non-obvious backup power supply for the detonator could certainly be a way to make it more dangerous. — alex.forencich, Sep 17 '14 at 19:17

tcrosley · Answer 6 · 2014-09-17T09:39:27.543

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In terms of how this can be done electrically, here are some examples:

1) The powered wire may be connected to a normally closed relay. so when power is removed, the relay disengages, causing the normally closed contact to close, setting off the bomb. This however is not a very good scheme if the bomb electronics are powered by a battery, since the continuously-powered relay would drain the battery.

2) The powered wire could be connected to the base of a BJT or the gate of a MOSFET, with a bias voltage set up so when the wire is cut, either the BJT or MOSFET switches on, setting off the bomb.

3) The powered wire is fed into the input of a microcontroller, with a pull-down resistor so the input won't be floating when the wire is cut. When the micro sees the input go to 0, then it sets off the bomb.

edited Sep 17 '14 at 09:39

answered Sep 17 '14 at 05:11

tcrosley

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I like how the "simplest" option involves an entire microprocessor. That's just, what, 10K+ transistors for "simple"? – Connor Wolf Sep 17 '14 at 07:07
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That's how you distinguish programmers. Writing few lines of code with COTS is what we call simplest. – Mazyod Sep 17 '14 at 08:23
@ConnorWolf Granted, not any simpler than a BJT/MOSFET solution. Edited. – tcrosley Sep 17 '14 at 09:39
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@Mazyod - There is a non-trivial distinction between simplest *to implement* and simplest in terms of actual system complexity. MCUs make the former very easy, by commoditizing the latter massively. – Connor Wolf Sep 17 '14 at 09:43
@ConnorWolf yeah, I know. That's why I didn't mention you in the comment, it's wasn't about what you said. It was a general statement, and it's true (see tcrosley's profile on programmers exchange). – Mazyod Sep 18 '14 at 09:36

score 10 · Answer 7 · answered Sep 17 '14 at 10:30

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If the bomb detonator wire is active low, then cutting the wire could cause it to explode.

For example, in the case where a bomb is rigged up to a dead man's switch, while the person is holding the switch, a high signal will be sent to the bomb and the bomb will not go off. If someone cut that wire with a pair of side cutters, the high signal to the bomb will change to floating and the bomb may go off.

Note that there may only be one wire that would do this, only the detonator wire, and that a floating signal may not be enough to set the bomb off, but it is a conceivable scenario that cutting the right wire in the right system could cause a bomb to go off.

answered Sep 17 '14 at 10:30

stanri

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Ah, so it's called a dead man's switch so that if the person holding the switch dies, the bomb is sure to go off, making sure he dies. Very clever! – Marc van Leeuwen Sep 17 '14 at 12:44
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@MarcvanLeeuwen I think the idea behind dead man's switches is that they also make sure that the person who killed said man (along with anyone and anything nearby) dies along side him when the bomb blows up. That is, if you shoot the guy holding the dead man's switch, the bomb goes off and you die, too. – reirab Sep 17 '14 at 19:20
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@reirab: although in practice you might have to either use a *really* big bomb, or settle for only being sure to kill assailants who stab the guy to death. Depends on the longest LOS to him, I suppose. – Steve Jessop Sep 17 '14 at 19:44
Right, yes, it would depend heavily on the longest LOS as well as factors such as how well-sealed the room happened to be, how much shrapnel the bomb would throw up in the air, etc. Obviously, if you shoot the guy with a sniper rifle from a rooftop 2,000 ft. away, you're probably ok unless the bomb happens to be nuclear or maybe a MOAB. If you happen to be in a room that isn't particularly large with the guy when you shoot him, though, then you're probably in trouble. – reirab Sep 17 '14 at 20:15

hkBattousai · Answer 8 · 2014-09-19T08:24:09.983

A typical will explode if you cut the power structure:

enter image description here

As far as I know, bombs usually have a heater device inside them, which increases the temperature around a tiny point to start the chain chemical reaction of explosion.

In the circuit above, when the power is on, - input voltage of the opamp is higher than the + input of it; thus the output of the opamp is low, and the MOSFET is turned off (i.e.; the heater is cold). If you cut the power, the 1000uF capacitor will still have enough energy in it to keep the circuit alive for a while and drive the heater. The - input voltage will drop lower than the + input voltage, which will make the opamp output high. The MOSFET will turn on and heater will heat the explosive.

I don't know the chemical background of bombs, but I think heating up a tiny point will be enough to start the chemical reactions. C₁ and C₂ are for noise immunity. C₁ can have a smaller value to lower the R₁C₁ time constant though. A low power opamp will drain negligible current from the capacitor. So it is all up to designing the heater appropriately.

Normally, yes, a bomb wouldn't be able to explode after cutting the power. But that makes it easier to deactivate. They add a brownout detector circuit like this to detect if the power is cut and explode in case to make the things more dangerous.

However, the bombs like in those movies usually use a microcontroller with on chip brown out detection feature. So, they don't use a circuit like this. The microcontroller detects the drop down in the supply voltage and calls the brown out interrupt. That interrupt triggers the heater device.

I doubt you have enough power in that capacitor--you can run a low power current through a blasting cap to test the wires without risking setting it off. I don't know what's needed, though. — Loren Pechtel, Sep 18 '14 at 20:50

score 6 · Answer 9 · answered Sep 17 '14 at 11:14

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There's lots of ways to make a circuit or mechanism that would always detonate the bomb unless a certain shutdown process was gone through.

If you charged something like a large capacitor or automotive ignition coil slowly and wired it to the detonator, cutting the power could induce a spark or other ignition, whereas a careful discharging of the device would render it safe.

It's similar thinking, design-wise, to "fail safe" devices but with a different end result.

answered Sep 17 '14 at 11:14

John U

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More of a "fail unsafe" in this case, I suppose. – reirab Sep 17 '14 at 19:23
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@reirab The term for that is [Fail-deadly](http://en.wikipedia.org/wiki/Fail-deadly). – Nick Alexeev Sep 19 '14 at 20:10

score 5 · Answer 10 · answered Sep 18 '14 at 15:49

Imagine detonator where electomagnet keeps firing pin in armed state. If you cut wire electromagnet release firing pin causing bomb to explode. So in this case you have to stop clock without disconnecting power to whole device to gain time to disarm da bomb completely or annihilate it in safe place

score 2 · Answer 11 · answered Sep 17 '14 at 21:55

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Fact of the matter is; when you have an explosive device like in the movies, they are in a normally closed circuit (like a home alarm system. Once the circuit is opened it releases what ever energy was stored in the system.

answered Sep 17 '14 at 21:55

Factulas

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Why would clipping a wire cause a bomb to explode?

11 Answers11