# Why would clipping a wire cause a bomb to explode?

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:

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

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.

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 - Warning: TV Tropes link Sep 17 '14 at 7:50
• 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 ;_; Sep 17 '14 at 15:26
• Two words: NOT gates. Sep 18 '14 at 3:33
• It's always the BLUE wire! Sep 18 '14 at 16:47

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

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.... Sep 20 '14 at 2:16

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.

• +1 It's ISO-0NO-1981, more colloquially known as the "oh no" standard. ;) Sep 17 '14 at 5:05
• @DrFriedParts: I cannot identify to which degree this is a joke. Are there any publications anywhere explaining it? Sep 17 '14 at 9:15
• @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. Sep 17 '14 at 14:31
• @DrFriedParts - AKA - Yellow then Orange = Kill Off - the short hand mnemonic is YOKO. Sep 17 '14 at 14:32
• @placeholder best answer ever Sep 17 '14 at 21:18

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.

• 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. Sep 17 '14 at 9:20
• 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. Sep 17 '14 at 10:20
• @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. Sep 17 '14 at 15:07
• 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. Sep 18 '14 at 6:17
• 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. Sep 18 '14 at 10:31

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.

• 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. 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) Sep 19 '14 at 5:23
• @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. Sep 19 '14 at 5:53
• Enter the THOR, man-portable IED Jammer. I think it jams GSM as well, however I don't know how much it's used. 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

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. 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. Sep 17 '14 at 19:17

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.

• I like how the "simplest" option involves an entire microprocessor. That's just, what, 10K+ transistors for "simple"? Sep 17 '14 at 7:07
• That's how you distinguish programmers. Writing few lines of code with COTS is what we call simplest. Sep 17 '14 at 8:23
• @ConnorWolf Granted, not any simpler than a BJT/MOSFET solution. Edited. Sep 17 '14 at 9:39
• @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. Sep 17 '14 at 9:43
• @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). Sep 18 '14 at 9:36

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.

• 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! Sep 17 '14 at 12:44
• @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. Sep 17 '14 at 19:20
• @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. 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. Sep 17 '14 at 20:15

A typical will explode if you cut the power structure:

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. Sep 18 '14 at 20:50

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.

• More of a "fail unsafe" in this case, I suppose. Sep 17 '14 at 19:23
• @reirab The term for that is Fail-deadly. Sep 19 '14 at 20:10

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

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.