Improvised Initiators + Detonators
All of the previously described low and high-explosives require an initiator or detonator for ignition. It is possible for the operative to produce initiators and detonators which are just as safe and reliable as commercially available ones.
Model Rocketry Match
Possibly the simplest initiator is what is know as a model rocketry match. This is a small wire with a blob of combustible material on the middle of it. It is design to be placed into the end of a model rocket engine with both ends of the wire sticking out and then ignited by placing a current of 9 volts through the wire. These matches are cheap and easy to use but you must make sure that the match itself is in intimate contact with the powder charge of your bomb. Using a setup such as that used for the lightbulb squib will make the match more reliable. A small model rocket engine, initiated with this type of match, would also make a very positive initiator for any low explosive bomb or for an incendiary device.
Burning Cigarette Delay
This is the simplest and least reliable type of delay mechanism. A book of paper matches is attached to a cigarette with tap or elastic bands. The cigarette is lit and left to smolder until it ignites the matches.
Cigarette companies have spent significant time and resources in developing cigarettes that won't go out if left unattended (so that the smoker will have to light another one upon return). Burn rates and reliability are different for each brand so do some tests if accurate delay times are required.
This device is perfect for arson attacks upon targets of low value such as mail-boxes.
A small lightbulb and a length of tubing are required to make this squib. Solder two wires to the contacts at the base of the lightbulb and then test the bulb with a battery to be sure its working. Carefully file a small hole in the tip of the bulb. Fill the bulb with a finely powdered, fast-burning, low explosive. Seal the hole with a drop of wax. Find a short length of cardboard, plastic or metal tubing which the bulb will fit snuggly into and glue the bulb into one end. Fill the tube with a low-explosive powder. Place a small wad of paper on top of the powder and then seal the end of the tube with wax or epoxy. You have just created a small explosive device so handle it accordingly. This squib will ignite even the most difficult to light low explosives such as Ammonpulver.
The best location for a squib is in the center of the low explosive filler. This step alone will decrease the burn time of a low explosive bomb by 50% and thus increase the power of the blast.
An overwhelming number of different detonator designs have been produced over the years. The IRA tends to use one improvised from a piece of 1/2" copper tubing containing 2.5 grams of mercury fulminate, ignited by a flashbulb. Detonators built from plastic tubes are being produced presently and the Soviet army was using cardboard tube detonators as early as WWII. Once you understand the basic principles of detonation it will be a simple thing to produce detonators in any configuration you choose.
As always I will attempt to keep things as straightforward as possible. The improvised detonator I will describe is produced from a spent .223 cartridge case though just about any piece of small tubing will do. It will be equivalent to a commercial #8 blasting cap, both in power and physical dimensions. The base explosive will be picric acid and the primary explosive will be HMTD. There are a number of different primary explosives that could be substituted but HTMD is the easiest to produce and is made from readily available materials. A word of warning, however, HTMD is not stable at elevated temperatures so dets made with it must be protected from heat. It will also corrode the cartridge case if stored for any length of time. Corrosion can be eliminated if the inside of the cartridge case is coated with a plastic or silicone sealer.
Hexamine - These are the small fuel blocks used in backpacker's stoves. They may also be found in surplus stores as army surplus fuel pellets. The ones you want are the small round ones, about the diameter and thickness of a Lifesaver candy. Scrape off the outer wax coating. The larger commercial packages hold eight small blocks weighing a total of 6 1/2 oz. They contain a purer grade of Hexamine than the pellets and are actually cheaper than buying the bulk chemical on a pound-per-pound basis.
Citric Acid (sour salt) - Available at some supermarkets in the home canning section. Citric acid is used to adjust the pH of home canned products.
6% Hydrogen Peroxide - Hair Bleach, found in drug stores everywhere. This is not the same as the 3% antiseptic solution.
Pour 45ml (9 tsp.) of hydrogen peroxide into a small glass jar. Add 2 1/2 tsp. of finely powdered Hexamine in three portions. Stir vigorously between additions to make sure that all of the powder dissolves. Add a little more peroxide if necessary. Place the jar in a basin of cold water or in the refrigerator for 30 minutes.
Remove the jar from the cooler and add 4 1/2 tsp. Finely powdered citric acid in five portions. Stir vigorously between additions as before.
Let the jar sit overnight (8 to 24 hours) at room temperature. White crystals will have formed in the bottom of the jar. Filter the solution through a coffee filter. Rinse out any crystals sticking to the jar with a little cold water. Air dry the crystals on the filter and scrape into a secure plastic container. A 35mm film container is ideal for this purpose. Use care when scraping the crystals off of the filter because they are sensitive to shock and friction.
An igniter match is similar to the Lightbulb Squib. Be sure that the bulb is small enough to fit into a the piece of 1/4" tubing described later. Using a nail file or emery board, file a small hole in the tip of the bulb. Fill the bulb with whatever finely powdered propellant you have decided to use. Seal the hole with wax, glue, silicone or a small piece of tape. Obtain a length of 1/4" tubing (any metal tubing will do) and cut a 3/8" length. Cut one end with a crimping cutter and the other end with a sharp cutter. A standard pipe cutter can be made to perform both of these tasks if you're careful; simply tighten up the cutter as far as it will go before cutting the crimped end and keep the pressure very tight throughout the cut. For the sharp cut use a very gentle cut with just enough pressure to make progressive scoring on the tube until its cut right through. Fit the bulb into the tube with the glass end just slightly protruding from the crimped end of the tube. Fill the gap between the tube and bulb body with epoxy or silicone and let it dry.
Prepare the cartridge case by sawing off the rim portion and using a file, remove a bit of material from the base of the cartridge. This will give the case a more uniform thickness reducing the chances of a misfire. Force a tapered 5/16" metal rod down the case mouth to enlarge the opening. Plug the primer hole in the base with a bit of epoxy and let dry. Spray plastic sealer inside the case to prevent corrosion and let dry.
Be sure that the picric acid base charge is very dry before loading. Dry it at about 80* C for two hours before loading. This can be accomplished with a heat lamp or a simple light bulb, be sure not to overheat. The HMTD should be dried at room temperature or slightly above, overheating it may cause it to become inert.
Pour half of the 1 gram picric acid base charge into the case. Use a 5/16" wooden or plastic rod to carefully press it into the bottom. Using a bench vise to slowly force the rod into the case will make a suitable substitute for a proper detonator press. A modified single stage reloader's press would also make a suitable press. Do not twist, tap or pound the rod in any way. A plastic bucket or a heavy cardboard box should be placed over the press in case of an accidental explosion, this will happen from time to time but the covering will protect you from injury. Add the second half of the base charge and press as before. Pour .75 to 1 gram of the HMTD into the case and press as above. Add a few grains of black powder to the top of the primary charge and then carefully slide the igniter match assembly into the case mouth and seal with epoxy. When the epoxy is dry, spray the entire unit with plastic sealer to waterproof it. The detonator is now complete and ready to use.
When using this det try to use more voltage than the bulb calls for. This will cause the filament to glow hotter and faster before it burns out providing more certain ignition of the propellant.
Timers And Delay Mechanisms
Probably the simplest delay mechanism is the slow-burning fuse. The materials and skills required to assemble this type of delay are quite basic and the finished product is very easy to use. The drawbacks to this type of delay include the fact that smoke emitted by the burning fuse may betray the location of the device.
A pyrotechnic fuse can be made with a length of string made from natural fibers, gunpowder or any other low-explosive powder, and some glue. Simply apply a bit of glue to the string and then roll it in the powder. Be sure to get plenty of powder to adhere to the glue and then allow it to dry. Once dry the fuse can be made more durable and moisture resistant by applying a thin coat of spray paint. The operative will have to experiment to determine delay time for these improvised fuses.
A sparkler can also be used as a pyrotechnic fuse. The sparkler burns at a consistent rate and is nearly impossible to extinguish once lit. The only drawback is that these devices are a bit difficult to ignite
Lead Break Fuse
This type of fuse has been in use since WWII and has proven itself to be rugged and reliable. A cocked striker, or firing pin, is restrained by a short length of lead solder wire. Pressure from the striker springs pulls the top of the striker shaft against the solder wire, slowly cutting its way through. When the solder can no longer restrain the load of the spring, it releases the striker to impact the primer and detonate the charge.
The length of the delay is determined by three factors:
- Spring Power - This is, of course, is self-explanatory. If you press harder against something you are cutting, it cuts faster.
- Solder Type - Solder wire varies in thickness and composition. Be sure you have conducted tests on the type of solder you intend to use before trying it with a real bomb.
- Temperature - In cold weather, solder wire hardens somewhat and will deliver a longer delay than one at room temperature. Extremely high temperatures, conversely, will cause the wire to soften which will shorten the delay.
12-penny duplex nail (striker)
5/16" metal tube, 3" long (fuse body)
#6 washer (shear pin support)
1/4" x 1 3/4" spring
1/4" x 1 1/2" steel bolt (primer/detonator assembly)
Small rifle or pistol primer
Blasting cap or improvised detonator
Coat hanger wire (arming and safety pins)
Solder wire (shear pin)
- Striker - Saw the head off the duplex nail and cut off any excess length. Chuck the nail in a drill, lock the trigger on and use a file to grind it into shape. Drill two 3/32" holes about 1 1/4" up the shaft from the tip at 90 degree angles to each other and 1/8" apart. File off any burrs. Chuck the nail back in the drill and polish it with a piece of emery paper until it is slick and smooth.
- Fuse Body - Cut off a 3" length of the 5/16" metal tubing. The best tool for the job is a pipe cutter. Purchase a cheap one and dull the cutting wheel with a file so that it will crimp tubing quite a bit before it cuts through. This will create a secure seat for the spring. Drill a 3/32" hole through the tube about 1/2" from the bottom (uncrimped) end. This will hold the positive safety pin. Remove any burrs from the tube.
- Primer/Detonator Assembly - Saw the head and all but 1/2" of the threads off the bolt. File both ends flat. Using a 11/64" bit, drill a hole in the center of the bolt (unthreaded end) just deep enough to seat the primer, no deeper. Next, drill a 3/32" hole in the center of the first hole all the way through the remainder of the bolt. This is not as easy as it sounds, so be sure to have extra bolts and drill bits handy. Chuck the bolt in the drill and use a file to grind down the threaded portion until it will easily slide into the mouth of the blasting cap. Reverse the bolt in the drill and, using the edge of the file, cut a groove into the side, about 1/4" down from the primer end. Seat a primer into the larger hole using a vise padded with a piece of wood. Do this slowly a carefully, and be sure to wear eye protection. Paint around the primer with a sealer, such as nail polish, to waterproof it.
- Shear Pin Support - Ream the center hole of the washer with an 11/64" drill bit.
- Apply a light coating of oil to the firing pin and spring, then slide them into the tube. Use a 1/4" rod to compress them until the striker shaft protrudes from the end of the tube. Slide the washer over the shaft and slip a short piece of coat hanger wire (arming pin) through the lower hole. Release pressure on the rod. The striker will be retained in the tube by the arming pin.
- Slip a length of solder wire through the upper hole and pull its ends down the sides of the fuzz body. Secure it in place with some tape.
- Slide the primer/detonator assembly into the tube. Estimate where the groove is located and use the dulled tube cutter to crimp it into place. Add a drop of superglue as security.
- Cover one of the positive safety pin holes on the fuse body with a short piece of tape. Pierce the center with a straight pin. Push the safety pin through this hole and into place. The grip of the tape will retain it.
- Pour a small amount of finely powdered low-explosive into the blasting cap and crimp or tape it into place. The fuse is now complete.
- Insert the fuse into the explosive charge. Withdraw the arming pin. The striker shaft will snap down onto the solder wire and slowly begin cutting through the solder wire. If the solder fails to hold, for any reason, the striker will be caught by the positive safety pin.
- If the solder holds, withdraw the safety pin.
- The fuse is now fully armed.
Clockwork Electrical Delay
The use of mechanical (analog) clocks and wristwatches is a common and effective method of delayed ignition. This fuse can be built from a wristwatch or an alarm clock. Both work on essentially the same principle, with some variation in construction. The basic idea of this fuse is to use the rotating hands of the clock to complete an electrical circuit and fire the bomb.
- Remove the plastic dial cover from the clock face.
- If a delay of less than 1 hour is required, remove the hour hand. If more than an hour is required, remove the minute hand. Remove and discard the second hand, if present.
- Scrape the finish off the leading edge of the hand where it touches contact #2. This will ensure a good connection.
- Drill a hole through the dial cover just big enough to receive the contact (#2). A small screw should be used as the contact, but if a wristwatch is used, it is best to use the end of the connecting wire as the contact. Tape or glue the contact in place. (NOTE: a blob of model airplane glue works well but avoid the use of cyanoacrylic "super-glues", as the capillary action of this type of adhesive has a tendency to coat the contact, effectively insulating it.)
- Replace the dial cover on the clock face. Check to ensure that the hand will touch the contact.
- Attach the other contact wire to the case of the clock. For alarm-type clocks, there is usually an external screw used for disassembly. This screws straight into the chassis of the mechanism and makes a perfect contact. For a wristwatch, the wire should be soldered to the case to ensure a good contact.
- Assemble the rest of the circuit as shown in the drawing. Make sure that the arming switch is in the OFF position. It is wise to use a light bulb to test the circuit to ensure its safety before a detonator is attached.
A collapsing circuit is a simple, cheap, and reliable delay that has seen long service with many terrorist groups. This circuit is so simple that it can be built by just about anyone… even if you don't have any electronics skills.
An electric relay is the heart of this delay mechanism. A relay is basically an electromagnetic switch. Current from a battery flows through its coil and generates a magnetic field, which moves a flexible contact towards or away from a fixed contact.
The two fixed contacts are known as "Normally Open" (NO) or "Normally Closed" (NC). When the relay is not energized, the moveable contact is positioned by spring tension against the NC contact. When the relay is energized, the moveable contact is drawn against the NO contact. One wire in the firing circuit is attached to the NC contact lead; the other is attached to the moveable contact lead (consult circuit drawing). When the relay is energized, the NC contacts are open and the firing circuit is incomplete. When the battery power degrades to the point where the magnetic coil can no longer hold the moveable contact against spring pressure, it touches the NC contact, completes the circuit, and fires the initiator.
The amount of time required for this action to take place depends primarily on the ohmic resistance of the relay coil and the type of battery used. Certain batteries, such as those used in camera photoflashes, have high voltage but can only sustain their level of power far a comparatively short length of time. Therefore, if one of these is used, the delay will be shorter. The more ohms a really is rated for the longer the delay will be.
Regular flashlight batteries of the same voltage rating will hold the contacts open for a much longer period of time than the aforementioned type, and alkaline batteries will hold them longer still. So, the combination of parts is dependent on the length of delay required. For a longer delay, use a relay with a high ohmic resistance and a battery made to deliver its rated voltage for long periods. For shorter delays, a relay with a low ohmic resistance and a battery of short current delivery is required.
With the proper combination, delays can be achieved ranging from about 15 minutes to several months. Its only drawback is that the accuracy is ballpark at best. Depending on temperature, battery condition, and other variables, the delay can swing as much as 25 percent or more either way. Use an electronic timer if split-second accuracy is required.
1 DC relay
1 relay battery
2 toggle or other spst switches
1 firing battery
1 electric initiator
1 green relay lamp
1 red relay lamp
Mounting board (wood, plastic or cardboard)
The voltage a relay is rated for can usually be exceeded by up to 50% without burning the coil out. For instance, a 6-volt relay can use up to 9 volts safely. This will usually give a longer delay, but this is dependent on the type of battery used. Conversely, if shorter delays are required, the relay can sometimes be run at a lower voltage. Testing is required to find out exactly how low it can go.
Shorter delays can also be produced by inserting a resistor into one of the power leads of the relay. Once again, some testing will be required to determine the approximate length of the delay. When doing this testing start with the lowest resistor available and go up in increments until you reach the desired length of time.
This type of delay can be built cheaply without the lamps and switches but they really make for a safe delay mechanism.
Flip switch A (relay) to ON position. The green lamp should come on with a constant, nonflickering light.
Flip switch B (firing) to ON position. The red lamp should NOT light.
Flip switch A to OFF position. The red lamp should now be lit.
Flip switch A back to ON position. The green lamp should light and the red lamp should go out. This shows that the circuit is safe.
If the red lamp is out, the detonator may be plugged in safely. The bomb is now armed. NOTE: If desired, the det jack may be replaced with two single strand wires, which are twisted around the leg wires of the detonator in standard fashion (Western Union pigtail splice).
This is probably the simplest type of mechanical delays. A clothespin switch is created by wiring each jaw of a regular wooden clothespin so that a circuit is completed when the clothespin is in the closed position. The clothespin is then held open by wrapping its arms with solder wire. When spring tension from the clothespin stretches the wire enough, the jaws close, completing the circuit and firing the bomb. The length of delay is determined by the thickness of the solder and the number of wraps used. Experimentation will allow you to determine the average length of delay. A piece of wooden dowel or a similar insulator can be used as an arming pin by placing it in the jaws and removing it to arm the device.
Straight Chemical Delay Fuse
This type of delay uses a corrosive liquid which eats its way through a barrier material and comes into contact with a reactive substance, producing heat and flame needed for initiation. The most commonly used corrosive is concentrated sulfuric acid, which is readily available. The most commonly used reactive is a chlorate-based composition, such as potassium chlorate and sugar or match heads. The materials that have been used as barriers are many and varied. In the past, materials such as paper, cardboard, copper sheet, rubber sheet (balloons, condoms, etc.), gelatin capsules, even Ping-Pong balls.
The chemical delay I will present here uses a gelatin capsule barrier glued to the end of a tube of sulfuric acid. It has the unfortunate shortcoming common to most chemical timers; temperature variation. A great deal of variation in delay times occurs at the low and high ends of normal temperature. At very low temperatures (less than 32 degrees F) it becomes inoperative. This is a real drawback, however this chemical delay can be counted on to function reliably and fairly accurately within median temperatures and it costs about a quarter to produce. Some experimentation will be needed to determine delay time.
Small Birthday Candles
Cut off a 3" length of the glass tubing. If you don't have a tube cutter, use a three-cornered file to scratch a line around the circumference of the tube. As you work your way around, the groove gets deeper and deeper. When the glass gets thin enough it will simply snap off.
Heat the end of a birthday candle and shove it into the end of the tube to a depth of at least 3/8". Cut off the excess.
Place the tube upright in a stand and, using an eyedropper, carefully fill it about 3/4 full with concentrated sulfuric acid. Wipe off the area around the neck.
Paint the area around the side of the neck with a thin layer of epoxy and quickly slide the capsule half over the end. After the adhesive dries, dab additional epoxy on any seams that look weak. Let dry.
Wrap a piece of electrical tape around the seam as insurance. Store the completed fuse in an upright position to prevent activation. This type of fuse should only be made up as needed and never stored for very long. Great caution must be exercised in transporting this type of chemical fuse.
In order to get this fuse to initiate an explosive device it must be fitted into a short piece of tubing with the reactive powder between it and the detonator. The detonator, obviously, must be of a non-electric type and will be plugged into the other end of the tubing.
Warning: As the acid erodes the gelatin membrane, it becomes thinner and thinner, and so the fuse becomes more and more sensitive to shock. Once armed, this fuse must not be disturbed as the result may be instantaneous initiation. Bomb squad personnel are aware of the hazards of attempting to move or defuse this type of detonator and are unlikely to even make an attempt.
Straight Chemical Fuse, Type II
This fuse works on the same principle as the previous one. In this example, the sulfuric acid is contained in a small glass vial. It is activated by crushing the upper portion of the tube, which breaks the vial and allows the acid to come into contact with the thin rubber membrane and eat its way through to the reactive material. The small glass vial can be obtained from perfume samples, made from modified lightbulbs or purchased at those druggie "head" shops. Whatever type of vial is used, its stopper must be well coated with wax in order to protect it from the acid. The size of the crush tubing depends on the dimensions of the acid vial.
Crush tube - Soft aluminum or copper tube with an inside diameter equal to the outside diameter of the acid vial, and about 3/4" longer than the vial.
Rubber membrane - The best material to use is the end portion of a long balloon. Avoid using a condom as the lubricants and spermicides they are coated with will prevent the acid from doing its work in a reliable way.
Support tube - Hard metal tube with an outside diameter the same as the inside diameter of the crush tube.
Adapter sleeve - May be needed if commercial blasting cap is used.
Acid vial - see above.
Reactive Material - Chlorate powder or match heads.
- Cut the crush tube to the appropriate length.
- Cut about 1" off the closed end of the balloon and slip it over the end of the crush tube. Make sure it fits snugly. Secure with a strip of tape.
- Slide the acid vial into the crush tube and secure it in place near the top with a drop of epoxy. When dry, plug the top of the tube with a blob of epoxy putty or auto body filler.
- Prepare the adapter tube from a larger piece of metal or plastic tube. It must be about 2" long and have an inside diameter that will snugly (not too tight) accept the crush tube with the rubber membrane in place.
- Slide the crush tube, rubber end first, into the adapter tube to a depth of about 1". Wrap a piece of tape around the tube to secure it in place.
- Fill the detonator's open end with reactive powder or match heads, and slide it into the open end of the adapter as far as it will go. DO NOT FORCE IT! Tape it into place. (Note: It may be necessary to sleeve the det with a short piece of tube to make it fit into the adapter. Glue the det to the sleeve with epoxy or superglue. Add a strip of tape as a sealer.) The fuse is now complete.
To use, place the detonator into the explosive mass. Make sure that the top of the tube is pointing upward. It will not function reliably in any other position.
Flatten the upper portion of the tube with a pair of pliers. The fuse is now armed. The sulfuric acid will eat its way through the rubber membrane and contact the reactive material, which will flash on contact and ignite the detonator.
Chemical/Mechanical Delay Fuse
This fuse is based on the same principle used in most military "time pencils." A striker or firing pin is restrained by a thin wire or line. This wire is wrapped at one point by a pad of absorbent material. A corrosive chemical is soaked into the pad and begins attacking the wire. After a period of time, the wire is unable to retain the load of the striker spring, breaks, and the striker impacts the primer. The primer in turn ignites the detonator, which detonates the charge.
The corrosive chemical is usually contained within a small glass vial, which the user crushes to activate the fuse. While this makes for handy operation, it poses problems for the improviser. First, it requires the machining of a two-part tube - one end of a hard metal to support the spring/firing mechanism, and one of a soft metal, which may be crushed by the fingers and which must be well sealed to prevent leakage. Second, the corrosive must be sealed into small, fragile glass vials that must fit into the narrow tube. This may all be more than the average operative cares to go through to obtain a delay of this type, however the finished product is an accurate and reliable delay mechanism.
The basic design may be modified for use with different solvent or corrosive and wire combinations. The solvent can be carried in a small plastic squeeze bottle such as is used to dispense nasal spray. The fuse is activated by squirting the solvent through the arming port onto the absorbent pad.
The illustrated example was built from 5/16" stainless steel tubing, but many different types of tubing or pipe (metal, plastic, etc.) may be used. The only alterations in the basic design will be due to the dimensional differences in the tubing used.
This version of the fuse uses a liquid solution of ferric chloride to corrode a copper restraining wire. This solution is readily available from electronics parts suppliers, where it is used to etch copper circuit boards. It is quite cheap, about $15 a gallon. The solution is soaked into the absorbent pad and begins eating its way through the restraining wire. The length of time required for breakage to occur depends on the temperature, strength of the solution, and the thickness of the wire.
As purchased, the ferric chloride is a saturated solution. This means that the water contains as much ferric chloride as it can hold. Adding water will weaken the solution and so extend the time delay. As always, test to determine the length of delay.
Firing Pin - The firing pin is made from a 12-penny duplex nail. The head and excess shaft length are cut off and the nail chucked into a drill. A file is used to grind the nail head down to the desired size and shape. The firing pin and spring fit together closely, eliminating the need for supporting spacers to keep it centered with the primer.
Absorbent Pad - The absorbent pad is a small wad of packed cotton.
Tube - The end of the tube is sealed with a plug of epoxy or auto body filler. On top of this filler is a short steel pin which serves as an anchor for the restraining wire. The wire is tied tightly to this pin before the epoxy is pressed into place.
Adapter - The primer/detonator adapter is identical to the one used for the lead break fuse.
Fuse Body - The fuse body is cut from a length of 5/16" tube, about 3" long. The dull cutter is used to crimp the tube about 3/4" down from the top. The arming port is a 1/8" hole drilled is the side just above the crimp.
- Attach one end of the copper wire to the firing pin. Give it a couple of twists and add a drop of solder to keep it from untwisting.
- Slide the spring over the firing pin and wire. Give it a light coat of oil and slide the whole assembly into the fuse body.
- Pull the wire through the other end. Pack the area above the crimp with cotton until it is about 1/4" from the top.
- Pull the wire up tight, compressing the spring. Wrap it around the anchor pin and cut off any excess. If there is excessive slack, it can usually be tightened further by rotating the pin a couple of times. Let the pin rest on the top of the tube.
- Fill the open space above the cotton with epoxy or auto body filler.
- Assemble the primer/detonator adapter and install in the tube as in the lead break fuse.
SCR Modified Electronic Clocks
There are many commercial time pieces on the market today that may be modified for use as electronic time-delay fuses. Travel alarms, countdown timers and digital alarm watches are the three main types encountered. All are adapted using the same basic mechanism - an SCR (Silicon Controlled Rectifier) switching circuit, although the construction details will vary slightly due to the differences in the physical form of the time piece.
The SCR is an electronic switch that may be closed by the tiny electric pulse generated by the alarm buzzer of the time piece. An SCR has three prongs - power in (from the battery), power out (to the detonator), and gate (to the alarm buzzer). Refer to the drawing for details.
When the time piece emits its pulse to the SCR's gate prong (A-1), it closes the circuit, routing the power from the positive side of the battery to the detonator. The circuit drawing contains two optimal accessories - a momentary switch (B) and a power lamp (D). These are not entirely necessary, but they will enhance both the safety and ease of use of the fuse.
The momentary switch serves to cut the power from the battery to the SCR. This is necessary because the SCR will not reopen until this power is cut, even though the pulse from the time piece has been interrupted. Any type of on-off switch may be used, but a momentary is easier to use and, usually, smaller.
The power lamp serves to indicate whether there is power flowing through the firing wires that lead to the detonator. It is very important to know whether the detonator is being connected to a live power source, considering the consequences if it is (instant detonation).
The source for these time pieces can be almost any type of variety store - supermarkets, drugstores, electronics hobbyist stores, or even auto parts stores. These units are produced very cheaply and can be purchased for as little as $2. It will be necessary in many cases to replace the batteries with fresh, reliable ones.
Lets take a closer look at the three main types.
Travel Alarm - These types are usually pretty small, about 2"-3" long and maybe 1/2" thick. If small components are used, all of the additional circuitry (with the exception of the firing battery) can be fit inside of the case. It functions like an alarm clock, so the current time of day as well as the alarm time (detonation) desired must be set.
Digital Alarm Watch - This is the smallest of the three and functions as the previous one does, i.e., alarm clock fashion. A wire is run from the alarm contact through the watch case to the SCR. The hole in the case may be sealed if the watch is to work, and the short length of wire concealed under a piece of electrician's tape. The SCR circuit must be housed separately, as there is no room in the watch case to house the components.
Countdown Timer - This is a modernized version of the old kitchen timer and is the best of the three to use. It is compact but still large enough to house all of the circuitry including the firing battery. It is also the simplest to set up.
The alarm contact varies with the type of time piece used. Watches generally use two small metal tabs that press against a flat disc to produce the alarm sound. The travel alarms generally use the same disc system, but the contacts are a pair of tiny brass springs. The countdown timer uses two wires to the buzzer, which may or may not be of the disc form.
Open the case on the time piece and expose the alarm buzzer. Test with a multimeter to find the positive contact on the buzzer. This is where the gate prong on the SCR is attached.
Assembly of the circuit is very simple and really requires no detailed instruction. As an added safety precaution, a safety switch should be added to the det wires. This would allow greater ease and safety in operation, as the detonator could be connected to the circuit and the operative could still set and test the timer with complete safety. Another safety measure is to connect the wires that ordinarily go to the detonator to a miniature stereo headphone jack. The leg wires on the detonator would be attached to a mating headphone plug. After starting the timer and finding the circuit safe, the detonator is plugged into the jack.
- Set the timer or alarm to the desired delay.
- Check the power lamp to see if there is power to the det wires.
- If all checks out OK, connect the detonator.
- Start the countdown.
Notes: Test the delay a couple of times by connecting a buzzer to the detonator wires. After the delay is complete and the buzzer sounds, the battery must be disconnected or the momentary switch depressed to break power to the det wires by resetting the SCR.
Short Delay Electronic Circuit
This device is easy to build, accurate, reliable and cheap. The parts are readily available from electronics supply stores and will cost under $5. Delays from under 10 seconds to about 3 hours can be obtained through different combinations of components. The length of delay is determined by the values of R-2 and C-1 (see chart)
Lets take a look at the circuit and its parts:
R-1 is a fixed resistor, valued at 4.7K. It never changes.
R-2 is another resistor and may be fixed, variable, or a combination of the two. The use of a variable resistor (potentiometer) will allow the time delay to be adjusted if necessary, within a certain range.
C-1, the capacitor, is a common electronic component. Increasing its value, either alone or in combination with R-2 (preferred), will give longer delays. Capacitors (and resistors) are found in many electronic appliances and may be scavenged from these sources.
Q-1 is a 2N3906 transistor. Many different types can be substituted, so consult with an electronics supplier if you can't find this exact one.
555 IC chip is one of the most popular ICs yet developed and may be found in many different types of circuits. Cheap and versatile.
Before building this or any other circuit, the operative should do some studying of basic electronics. When experimenting with electronics it is a good idea to use a solderless "breadboard" in order to test delays and gain experience.
This circuit is maxed out at 14M resistance and 1,000 capacitance. Remember to raise the value of the capacitor when you raise the value of the resistor to any significant extent. After final assembly, retest the delay time to make sure it hasn't changed. The particular soldering technique used may have added some resistance to the circuit, thereby altering the timing. After everything has been assembled and tested, it should be smeared with a "potting" material such as epoxy resin, which will serve to protect the delicate electronic components from damage.
Some nice touches to add to this circuit are a power switch, arming switch, and a firing lamp. This enables the assembled device to be carried and armed with maximum safety. Upon arrival at the target, the power switch is activated. If the red firing lamp doesn't light, the arming switch is flipped on. The bomb is now activated and will fire after expiration of its delay.
Long-Range Electronic Timer
This device is similar to the previous one in that it uses a 555 IC chip as its heart. Extensions in delay are obtained by adding on 4017 Decade Counters. The 555 is wired so that it will periodically emit a pulse to the 4017. After receiving 10 such pulses, the first 4017 will multiply the time delay of the previous chip by a factor of 10. As many 4017s as needed can be added, but as shown in the example, five decade counters will provide over two months of delay.
The basic time (the frequency of pulses emitted by the 555) is varied by altering the component values, as in the previous example. Do not be tempted to use large-value components (as in the previous example) to avoid using more decade counters. This is not exactly the same type of circuit, and large-value components may introduce instability into the circuit and cause the operative many headaches. A reliable delay is absolutely imperative. This calculation chart shows how to determine the delay from the values of the components.
F = (Frequency of output) 0.693 (Constant value for 555 IC) x 0.0001 (C-1, 100-uf capacitor) x 1,000,000 (R-1, 1M resistor) +2 x 4700 (R-2, 4.7K resistor)
F = 0.693 x 0.0001 x 1,009,400
F = 69.95 seconds
Each 69.95 seconds, the 555 will send a pulse to the 4017 chip and automatically reset itself. After 10 pulses are received ( a time delay of 699.5 seconds, or around 11 minutes, 39 seconds), it will send a pulse to the second 4017 and reset. After the second 4017 has received 10 pulses (a time delay of 116.5 minutes), it will emit a pulse and reset. And on and on, depending on how many 4017s are used. I've included a short list to show the possibilities for a given set of components. Remember: the components can be altered to achieve the required delay.
555 + 100-uf capacitor (C-1) + 1M resistor (R-1) + 4.7K resistor (R-2) 555 = 69.95 seconds 4017-1 = 699.5 seconds 4017-2 = 116.5 minutes 4017-3 + 19 hours, 25 minutes 4017-4 = 194 hours, 18 minutes, or approximately 8 days 4017-5 = Approximately 80 days
A Note On Batteries
Only fresh batteries should be used in any electronically powered explosive device. If any cold weather exposure (under 50 degrees F) is expected, alkaline batteries should be used. At temperatures below that the batteries should be insulated. Expanding foam insulation, the kind that is sprayed from a can, works well for this. This insulation will protect batteries and circuits at high temperatures as well.
Back to Mainpage