Justin A. Parr - Technologist

In movies, we often see technology myths that get repeated and propagated until they become “common knowledge”.

The technology of bombs and bomb-diffusal is no different.

 

 

Update: 3/14/2018

Added:  Bombs don’t beep or tick

Update:  4/10/2018

Added:  Radios do NOT trigger radio triggers

 

It’s a Trigger, Not a Detonator

Myth:  The bomber holds up a remote “detonator”, taunting the hero not to come any closer, or he’ll set off the bomb!

The bomber is actually holding a remote “trigger”, not a “detonator”.

Just like the trigger of a gun, a bomb’s trigger actuates the bomb, setting off the explosion.

The bomb’s detonator is a small primary charge, such as a blasting cap or percussion cap, that sets off the bomb’s main explosive charge.

In movies, we see many different kinds of triggers:

• A remote trigger actuates when the bomber either presses or releases a switch.
• A tripwire is usually strung across a walkway or stairway, so that it actuates the bomb when a victim pulls it by walking across it.
  • Sometimes in movies, and especially in video games, we see a laser tripwire.  Laser tripwires are much more complicated than they first appear, and thus they deserve a section of their own.  We’ll discuss them in detail later.
  • Tripwires are most often used with a claymore, which is a special type of military bomb that can be aimed ahead of time, and explodes so that shrapnel covers a specific arc in front of the device, leaving a (relative) “safe zone” behind the device.
    Claymores are used either autonomously as a “booby trap”, or manually, remotely-actuated from a defensive position.
• A trigger might include a timer or clock, that actuates when the timer reaches zero, or the clock reaches a specific time.
• A so-called “collapsing circuit” is a trigger wire that carries a small, live current all the time, and triggers the bomb when cut.
• Another common reference is a “mercury switch“, which is really an attitude sensor that triggers when the bomb is tilted or moved.
  • A mercury switch has two open-ended wires inside one end of a small glass tube, along with a blob of liquid mercury.  When tilted with the wires upward, gravity pulls the blob of mercury away from the wires.  When tilted past a critical angle, the blob of mercury slides to the other end of the tube, connecting the two wire ends, allowing electricity to flow.
  • Mercury switches were commonly used in older, analog air conditioner thermostats, where the switch is located at the end of a bimetallic spring.  As air temperature increases, the bimetallic spring expands, tilting the mercury switch, activating the air conditioner.
  • Older airplane attitude sensors used a grounded blob of mercury at the center of a small, circular, concave dish, surrounded by metal pins.  When tilted in any direction, the blob contacts one of the pins, allowing voltage to flow through that pin.  The airplane’s circuitry can then determine its pitch and roll based on which pin is carrying voltage.
  • In movies, you’ll occasionally see an improvised attitude sensor made from a weighted reed, ball bearings, or a pendulum.
  • Modern electronic devices, such as your favorite video game controller or smart phone, include a solid-state, digital attitude sensor.
• Some movies depict the bomb being triggered as the victim opens a box or panel, which would be accomplished using pin or reed switches.
• Sometimes the bad guy attempts to blow up an airplane using a barometric switch that reacts to the change in air pressure, which drops sharply as altitude increases.
• The movie bomb might also include a hidden trigger, such as a microfilament wire taped to the package, or a tiny pin switch on the inside of a panel.  The purpose of a hidden trigger, obviously, is to prevent the bomb from being diffused by exploding instead.
• Car bombs are often depicted as being triggered either by the ignition switch or the door handle.

Bombs might have multiple triggers, configured so that the bomb explodes in one of several conditions – for example, IF the timer reaches zero, OR the package is moved.

A bomb might have a multi-stage trigger, so that the bomb explodes when several conditions are met in series.  For example, the bomber presses a remote trigger, which then activates a 10-second timer (useless, but dramatic).  A better example is a bomb that’s designed to blow up an airplane at a specific location might have a barometric trigger that arms when the plane takes off, and starts a timer that reaches zero when the bomber assumes that the plane has flown a specific distance.

 

Radios Do Not Trigger Radio Triggers

Added:  4/10/2018

In Die Hard 3, the villain makes a comment about obtaining cheap radio detonators, that might get set off by police radio signals.

First of all, it’s a radio trigger, not a radio detonator.

The dialog makes it seem like talking on the radio might set off the bomb, but that’s just ridiculous.

Oversimplified:  When a radio trigger receives a specific radio signal, an electric circuit attached to the receiver triggers the detonator, detonating the bomb.

First, let’s consider that it takes more than just any radio wave to set off a radio trigger.

The trigger must be configured to “listen” to a specific range of frequencies, which is fairly simple.  However, we are constantly surrounded by radio signals all the time, in all sorts of frequency ranges.  So if the trigger were simply configured to listed to a “police frequency”, it would instantly detonate – not very useful.

So next, let’s consider proximity.  When we discuss a radio transmitter, power is the inverse-square of distance, so as the transmitter gets closer to the trigger, power increases exponentially.

So a clever bomber might think:  I can tune the receiver / trigger to a specific frequency, AND set a specific threshold,

The problem is that police radios (and many two-way systems) use an asymmetric system of communication.  The problem is that a tiny handheld radio has a small battery that can’t supply enough power to transmit a signal across the entire city, so there is a network of radio towers called base stations, that cover a large geographic area while allowing the handheld radios to operate using minimal power.  Since the base stations operate on the same frequency as the radios, proximity to any base station would instantly detonate the bomb.

What about systems that use different frequencies for the hand radios vs. the base stations?

A system like this uses frequency1 for the handhelds, and frequency2 for the base stations.  The thought process behind this is that the handhelds have less transmitting power, and therefore, having the handsets transmit at a lower frequency saves battery life.

Maybe the bomber could tune his bomb to listen for ONLY the frequency of the handsets, AND set a specific threshold.

The problem with this approach is that there are resonant frequencies.  A strong transmitter at the wrong frequency, near a piece of metal of just the right length might re-radiate a secondary radio wave at the target frequency.

To be sure, the bomber would have to know, in advance, that the target area was clear of the target frequency above a specific power threshold.

Even so, the bomber runs the risk that some coincidental activity, such as someone on a cell phone walking past a metal pole could cause a spike in the right frequency range, at the right threshold, even if no police radios are in the target area.

Because of this dilemma, real radio triggers use an encoded signal.

An encoded signal transmits on a specific frequency, but uses a specific sequence of power spikes.

A power spike is above the receiver’s threshold, while the space between spikes is below.  On the receiver’s side, this translates in to a specific sequence of high/low pulses, where a spike equals a pulse, but the space between a spike (called a low pulse) indicates the lack of a spike.  The receiver is is configured to trigger the bomb ONLY if a specific sequence of high / low pulses is received.

This can easily be done using a latch circuit – If the initial pulse is correct (high vs low), the latch moves to the next step in the sequence.  As each subsequent pulse is correct, the bomb eventually triggers.  Any incorrect pulse within the sequence causes it to reset back to the beginning of the pattern.

There are many variations on this theme.  For example, modern, digital circuitry and software-driven circuitry can easily be configured so that the transmitter has to send a digitally-encoded, encrypted code word.

On 4/8/2018, I watched an episode of Imposter, where the cop deduced that the suspect had built a bomb, and instructed the cops to “stay off the radio”.

• How did she know it was a radio trigger?
• How did she know what frequency would trigger the device?

 

Defeating a Radio Trigger

Radio triggers can be defeated in a couple of different ways.

• A frequency jammer emits white noise (random power values) within a specific frequency range.  The “real” signal to detonate the bomb would be lost within the noise.
• A super-strong pulse on the frequency used by the radio trigger can actually short out the radio receiver circuit, preventing it from receiving the signal to trigger.

 

 

A Word on Laser Tripwires

Popular in both video games and movies, a “laser tripwire” triggers a bomb, usually an anti-personnel device, when someone interrupts a laser beam.

The problem is that such a device is completely impractical and unreliable.

The reality is that a simple piece of fishing line is cheaper, more practical, and completely reliable.

 

How an Optical Coupler Works

Any laser trigger would be part of a family of devices known as optical couplers, where a light emitter is “coupled” to a light detector, and any interruption between the emitter and detector results in a signal to the electronic circuit.

This is exactly how a smoke detector works:  When smoke particles block an LED light, the detector can no longer detect it at a sufficient level, causing the circuit to trigger the alarm.

Optical couplers are used extensively in robotics and other electromechanical applications to very accurately detect the position of a component within a device – for example, to detect when the print head of a printer has reached the end of its travel, or to build a self-calibrating 3D printer.

 

Laser-Coupled Trigger

The problem with a normal light source is that its intensity drops significantly with distance – intensity drops inversely-proportional to the square of the distance from the light source.

If the light source is too far away from the detector, the light reaching the detector is too weak to be detected, and the detector remains in a constant open state (as if it was triggered).

Although an optical coupler only works over distances that are typically only a few millimeters, using a laser as the light source allows the distance between emitter (the laser) and detector to be increased significantly, without having to increase the emitter’s power.

This works because a laser emits coherent light – all of the photons are emitted at the same frequency and direction, so that its power remains constant over a much longer distance.

In movies, we often see a “laser tripwire” tied to the museum’s alarm system.  In this situation, the laser source has been carefully placed so that it aligns with the detector, or so that it bounces off of a reflector and back to the detector.

It has also been carefully mounted so that ambient vibration won’t cause it to trigger, and the laser’s output power has been calibrated based on the distance, to ensure that it does trigger when it’s supposed to.

So, there is a real-world application where specially placed, mounted, and calibrated lasers can be used as a trigger, but requires careful measurement, preparation, and installation that would take way too much time “in the field”, to use as an ad-hoc trigger for a mine or bomb.

 

Duke Nukem 3D

As depicted in such video games as Duke Nukem 3D, the protagonist (and sometimes the AI) is able to place what looks like a small C4 charge, with a laser emitter on any flat surface.

When the enemy crosses the laser’s beam, BOOM!

The problem with this type of device is that, unlike a laser-coupled trigger, it’s open-loop.

A coupler is closed-loop, because the laser emits the signal, which is then detected by the detector.  Interrupting the beam “opens” the loop.

With the Duke Nukem device, the loop is already open!

Assuming that the bomb has both a laser emitter and detector, it would still require a reflector in order to be “closed loop”.  It would require that the player places the bomb, then carefully places a reflector at the other end of the laser beam, to reflect it back at the bomb.  If the reflector is off by a few degrees, it simply wouldn’t work – thus requiring precision calibration.

You couldn’t just stick the bomb to a wall and walk away, as depicted in the video game.

OK, maybe it acts like an optical mouse – it self-calibrates and looks for motion.

Optical mice only work under specific conditions:

• There is very little distance between the optical coupler and the surface of the mouse pad
• Optical mice tend not to work on clear or smooth surfaces
• Optical mice tend not to work on surfaces that simply absorb the light emitted by the coupler

A self-calibrating laser trigger would have the same issues, plus, any vibration or change in ambient light levels would trigger the bomb.

Moreover, most lasers that could run off of a portable power source are not designed for continuous use – they would either overheat or drain their power source in a short period of time.

 

 

Cut the Red Wire! No, the Blue Wire!

Myth: At the beginning of “Lethal Weapon 3”, we see Riggs and Murtaugh argue, as Riggs decides which color of wire to cut.

Unfortunately, there is no “universal bomb-maker’s guide” that mandates a specific color scheme for a bomb’s wires!

Although most bombs with electrical / electronic triggers are DC, and there are definitely standard colors for DC vs. AC wiring, there is no rule or law that requires the bomb-maker to follow any particular scheme.  Maybe he likes yellow and pink.  Maybe he makes ALL the wires green!

To figure out which wire to cut, regardless of what color it is, you have to trace each wire in order to figure out its purpose.

It also helps to know the bomber’s intent.  For example, if he simply wants to blow something up, the bomb might consist of a digital timer that triggers an electric blasting cap as a detonator.  Or, perhaps the bomber is holding out for money or some other form of extortion – he will probably have included some kind of countermeasure, to prevent the bomb from being disabled.

On a “simple” bomb, you would expect to see wires that lead from a power source to the electronic timer, and other wires that lead from the timer to each blasting cap.

Although you might start by clipping the wires to the power supply, you never know if the bomber built in a capacitor or smaller battery that the timer can use to detonate the blasting caps if the main power supply is cut.

Maybe you should cut the wires for the blasting caps?  You’d better make sure you cut EVERY one of them – if you miss even one blasting cap, the bomb will still explode.  Also, since a blasting cap requires certain voltage and amperage levels to detonate, you never know if the bomber isn’t using low voltage and a small current in the blasting cap wires as a collapsing-circuit trigger, where cutting one blasting cap could detonate the others.

Maybe the bomb has extra wires that don’t seem to lead anywhere… each one could be a trigger or a dummy wire, simply put there to create confusion.

 

This is why the bomb squad doesn’t cut wires to disarm a bomb.

Instead, they send in a robot with a small explosive charge, that when detonated, propels a jet of water at supersonic speed through the bomb’s trigger, disrupting it, and cutting it to pieces in a fraction of a second before it can send enough current to the detonator.

Once the bomb is rendered safe, they either use a robot to drag the remains out, where it can be safely detonated, or they disassemble the remaining parts of the device.

 

Everyone Cut on the Count of Three!

Myth:  The hero discovers several triggers, any of which could detonate the bomb.  He recruits several reluctant volunteers, and starts handing out scissors.  He says, “we all cut on the count of three”.

One, Two, Boom!

Even the most crude digital circuit can react in a few microseconds (1/1,000,000 of a second), which is about 10,000 times faster than a human can react.

The average human reaction time is about 1/3 of a second, or about 300 milliseconds (1 millisecond = 1/1,000 ).

Even if one person cuts all the wires with one cutter, the fact that the cutter acts sequentially will cause the bomb to trigger.

Again, this is why no one cuts wires to diffuse a bomb!

 

 

The Timer Stopped at 3 Seconds

Another wire-cutting myth:  The hero cuts the wire, and the timer freezes, showing just a couple of seconds to spare!

This myth dates back to the late 60’s and early 70’s, before  you could go buy a cheap digital clock.

Although analog timer circuits are quite common, they are inaccurate.  The last thing you want, is for your bomb to go off early or late.

Digital timers are extremely precise, but back then, the technology didn’t exist as something you could go purchase at the store.

You had to build an oscillator circuit, possibly using a flip-flop transistor array, which then acts as a pulse generator in to an accumulator.

“Accumulator” is a fancy word for “counter”.  A timer has to have an encoder, in order to figure out how many clock pulses equals the desired time delay, and a decoder that translates the accumulator’s value in to a digital time display.  In a countdown timer, the accumulator starts at a given value, and each pulse subtracts one, until the timer reaches zero.  A simple transistor logic comparison then fires the trigger when the accumulator reaches zero.

All of these components, back in the Transistor Logic days, might be separate, individual circuit boards, interconnected with wires.

So if you’re REALLY good, AND the bomber didn’t booby trap the device, you could theoretically cut the pulse wire, thus freezing the accumulator at a specific value, and the decoder would continuously display the “frozen” accumulator value.

In modern electronics, a digital timer is one component.  It wouldn’t “freeze” with 2 seconds left.  It would be “on” or “off”, and the chances are good that if the timer is “off”, the bomb would already have triggered.

 

Bombs Don’t Beep or Tick

Added:  3/14/2018

While we’re on the subject of timers, in TV and movies, we often see them beep or tick.

Why would a bomber intentionally design a feature that allows the bomb to be located, or even detected at all?

In older movies, bombs are often depicted as some red sticks of dynamite or TNT attached to some wires and an alarm clock.

One of the earliest ways to delay a bomb’s explosion was to use a clockwork mechanism attached to a mechanical or chemical trigger.  When the clock hits a specific time, the bomb would be triggered mechanically, either by releasing a striker which in turn hits a percussion cap, detonating the bomb, or by releasing a small quantity of certain chemicals, which mix, become unstable, and then either detonate the bomb directly, or ignite the fuse to a traditional blasting cap.

Likewise, a cheap, mechanical alarm clock can be used as an improvised timer, by repurposing the alarm so that it completes a circuit, detonating the bomb electrically.

Anything with a mechanical clockwork would produce an audible ticking sound, but a clever bomber would simply insulate the bomb’s outer casing, so that the ticking is less audible.

However, there is absolutely no reason for a digital timer to produce a beeping sound.

Some hand-held countdown timers and lap timers might be configured to beep every so often, as a pacing mechanism.  So if the bomber repurposed a coach’s timer, maybe his bomb would beep, but again, there is probably an option to simply turn off any audible beeps.

My theory is that the first few movies to depict a bomb with a digital timer needed some way to replace the drama of a mechanical clock, slowly ticking down to zero-hour, so the director added a beeping sound for dramatic effect.

Why would the bomber intentionally add a feature that would result in the victim inadvertently detecting that the bomb exists at all, much less aid the victim (or the bomb squad) in locating the device?

 

A Big, Fiery Explosion

Myth:  Alas, the hero doesn’t manage to diffuse the bomb, resulting in a huge, fiery explosion.

In reality, what you most often see in the movies is a blasting cap inside of a can of gasoline or kerosene, resulting in a big, flashy fireball, but such a device would do very little actual damage to a vehicle or structure.

A real bomb either uses a high-explosive primary charge, which detonates very quickly, and is capable of cutting through rock or steel, or a low explosive such as gun powder, inside a pressure vessel designed to increase the bomb’s pressure and resulting explosive force.

All you see in real life is a puff of smoke and dust, and maybe a flash of light.  The next thing you sense is the shockwave.

 

Grenades Really Blow (Up)

Multiple grenade myths.

 

How a Grenade Works

When you pull the pin of a grenade, it releases the handle (also known as the “spoon”).

If released, the spoon flips up, over the top of the grenade, releasing a striker.

The striker hits a percussion cap, igniting a 4 to 5 second fuse.

When the fuse expires, it ignites a blasting cap, which ignites the high explosive within the grenade.

Until the spoon is released, the pin can be reinserted, rendering the grenade “safe”.

 

Pineapple Grenades

…haven’t been used since World War 2.

A “pineapple” grenade, also known as a “mark 2”, has a solid, metallic, oblong, serrated, waffle casing, which fragments when the grenade detonates.

The theory is that the grenade will separate along the serrations in the casing.

The problem with pineapple grenades is that they break apart in to several large pieces, and don’t fragment evenly.

If you see a pineapple grenade used in a modern movie, it’s an anachronism.  Any modern movie would use an M33 or M67 frag grenade.

 

M67 Fragmentation Grenade

Unlike the pineapple grenade, where the body of the grenade fragments in to shrapnel, the spherical M67 (and its predecessors, the M26 and M33) have a serrated, tightly-coiled spring inside a much thinner shell, which fragments in to shrapnel more effectively and reliably upon detonation.

The M67 is much more efficient, and thus, much more deadly, due to the use of the serrated fragmentation spring, a modern explosive charge, and a detonator that’s designed to ignite the primary explosive completely.

In addition, the timer is more consistent and reliable.

 

Grenade Used as a Booby Trap

Myth (Sort of):  We see the hero carefully place a grenade inside a cardboard tube, or against a door handle.  The bad guy opens the door, and BOOM!

Although a grenade absolutely can be used as a booby trap in any number of ways, releasing the handle (“spoon”) ignites a 4 to 5 second timer.

If the bad guy actuates the grenade, he still has about 4 seconds to react.  He could easily dive behind a barricade, or even kick the grenade out of range before it detonates.

Grenades absolutely CAN be used as a booby trap, with the caveat that they will still require 4 to 5 seconds AFTER actuation to detonate.

Examples:

• A grenade inside of a glass beaker or tube, with its safety clip removed and its pin pulled, prevents the handle from actuating the striker.  Breaking the glass frees the handle, releasing the striker.
• A grenade can be taped above a door frame, with a string tied to its pin, and the other end taped to the inside of the door.  Opening the door pulls the pin, releasing the handle, and initiating the 4 second fuse.  Anyone entering the doorway will be blasted from above or behind.
• Similarly, anything that moves can be used as a trigger to pull the pin, as long as the grenade is secured so that it can’t move, and such that there is sufficient clearance for the spoon to swing over the top of the fuse, releasing the striker.

 

“Baking” a Grenade

Many popular video games and movies depict the protagonist “baking” the grenade:  he releases the spoon, counts to 3, then throws.

The reality is that you might be a second too late!

There are many documented cases where a defective grenade detonated prematurely.

Rather than “bake” an M67 grenade, a better approach is to use an M68 “impact” grenade.

An impact grenade arms after 1-2 seconds (typical flight time), and explodes on impact.  A backup fuse detonates after about 7 seconds.

M68 grenades are denoted by a red fuse and spoon.

 

Here’s Your Grenade Back

Myth:  The antagonist throws a grenade.  The quick-thinking protagonist grabs it, and tosses it back.

As we’ve seen, a grenade has a 4 to 5 second fuse.

If the antagonist pulls the pin and immediately throws it, the flight time is about 2 seconds or more.

Human reaction time is about  1/3 of a second, leaving just a slim 1.6 to 2.6 seconds to grab the grenade and throw it back.

There might be enough time to dive away, or sweep the grenade away after it lands, but grabbing and throwing it would probably result in your hand and arm being vaporized.

 

Shooting a Grenade

Myth (sort of):  The antagonist throws a grenade, and the protagonist shoots the grenade, detonating it in mid-air, and the hero escapes safely.

Grenades contain a high-explosive charge, normally detonated by the grenade’s detonator – a fancy blasting cap.

High explosives can also be detonated by anything with sufficient kinetic energy, such as a high-velocity rifle bullet.

If the hero shoots the grenade with his trusty assault rifle, it’s likely to explode in mid-air, throwing deadly shrapnel everywhere, and he stands a good chance of getting hit.

However, handgun cartridges typically lack sufficient energy to detonate a high explosive, so if the hero thinks quickly and uses his handgun, it’s likely to disrupt the grenade’s high-explosive charge, resulting in NO explosion, and the hero escapes safely.

 

Hold This

Myth:  The bad guy hands the victim a grenade, pulls the pin, and walks away.  Eventually, the victim’s hand gets tired, and the grenade eventually slips from the victim’s shaking fingers, resulting in an explosion.

As long as the handle is not released, the grenade is safe.

Options:

• Use a toothpick in place of the pin
• Use a small piece of wire from a lightbulb, furniture, your purse, a brasiere, or anything you can find, to jam in to the pin hole, thereby securing the handle to the fuse.
• Use a small piece of fabric, tape, wire, or string to wrap around the handle, thus securing it from releasing the striker.
• Jam the grenade between the wall and a heavy piece of furniture, so that the spoon is trapped, and can’t actuate the striker.
• With the handle held tightly and immobile with one hand, use the other hand to slowly unscrew the fuse from the top of the grenade.  Once the fuse is removed from the grenade (or vice-versa), toss the handle-fuse assembly away.

 

 

Conclusion

There are many movie myths about bombs that we see perpetuated until people think they are true.  Some are perpetuated for dramatic effect, while others are simply perpetuated because that’s what the audience expects to see, due to the myth itself.

It’s important for the writers to use a factual basis for their work, rather than propagate the myths that we all seem to like to see in movies and video games, but that are simply not true.