British Boobytrap Mechanisms

A department of the Ministry of Defence, M.D.1, was tasked to develop many of the boobytrap devices used during the war.  They developed "Switches" that were used throughout the war and for many years thereafter.  The Second in Command of M.D. 1, Lt Col Stuart Macrae was responsible for developing a number of the switches, notably the pull and release devices.  Later in the war he also developed a combination switch. 

 Boobytrap mechanisms were also developed by "Experimental Station 6", a cover name for the SOE (Special Operations Executive) production facilities.  The SOE ordered large numbers of switches through the British Security Coordination in the US resulting in the American OSS adopting several British switches or developing their own from the British switches.

  trip mechanism 4   
  Trip Mechanism No. 4   
  Trip mech 5   
  Trip Mechanism No. 5 Pressure/Release   
  percussion igniter    
  Percussion Igniter, Mk.  3  
  Switch, No. 1, Pull, Mk. I

Initially known only as the "Pull Switch" this device was designed near the end of 1939 by Lt. Col. Stuart Macrae of M.D.1 (Military Department 1).  It came about because the only device the British had to set off a boobytrap with a trip wire was the standard service igniter, a device actually designed to replace matches for lighting a length of safety fuze.  The service igniter worked perfectly well for the purpose it was designed but left something to be desired as a boobytrap device.

Lt. Col Macrae in fact designed the pull switch from a shirt stud that worked in a way that he considered quite clever.  The stud had a detachable head that remained firmly in place normally but when a centre pin was pulled out could easily be removed.  The head was attached to a thin split tube with the end slightly bulged out.  The body of the stud was a hollow button with another small tube projecting out from it.  The stud was assembled by pulling out the centre pin as far as possible, pushing the split tube through the other tube until it passed through and pushing in the centre pin again.  Pushing in the centre pin expanded the split tube so that it would not pass through the other tube until the centre pin was pulled out again.  It was from this small device that Macrae got the idea of how to design the pull switch.  Once Macrae had the idea he designed the pull switch in an hour.  The prototype was made the next day and two weeks later MD1 was in full production of the switch.  It was a very successful design that worked perfectly from the start.  Over three million were made during the war at a cost of 2/6d.  It is a measure of its success that it was not redesigned throughout the war but remained in its original form.

The official description of the switch was:  "The Pull Switch is a device to be used in conjunction with a trip wire for firing a mine.  It is particularly useful for booby trap work.  Designed to operate when a direct pull of about 4 lbs is applied to the release pin it fires a percussion cap in a holder exactly the same as that supplied wit the Mk. III Service Igniter.  With the cap holder in position the switch measures approximately 4" overall by 5/8" in diameter and weighs 2 ½ oz.  It consists of a head into which is screwed a housing tube containing a plug with a restricted opening.  At the other end of the tube is a screwed collar for attachment of the cap holder.  Inside the housing tube is a striker head attached to a hollow spindle, which runs inside a compression spring.  The end of the hollow spindle carries a small head which is split, so that when it is compressed the diameter is reduced.  When this split head is forced through the small hole in the housing tube plug, a spring loaded release pin at once enters and expands the head so that it cannot return, although it is now under the influence of the compression spring.

 A safety pin can be inserted through the head and release pin so that the latter cannot be disengaged from the split head and it is impossible for the striker to fire the cap.  Without a loading spring on the release pin, a pull of 1 lb will free the striker.  The pull required can therefore be varied by having a stronger or weaker loading spring.  The standard spring used calls for a 4 lb pull."

The device consists of a brass tube threaded on both ends.  A threaded collar on the bottom end holds a standard fuze adapter with percussion cap.  The upper end has a threaded housing that is fitted with the release pin and spring.  A brass plug with a hole through the centre is fitted into the top end of the brass tube before the housing is screwed on.  A striker and striker spring fit in through the bottom of the tube.  The split stem of the striker fits through the plug up into the housing until the release pin penetrates and expands the striker stem.  A safety pin fits through the housing and release pin preventing it from moving.  A split ring is fitted through a hole in the end of the release pin so a trip wire can be easily attached.  An anchor bracket may or may not be fitted by removing the collar and slipping the bracket over the tube and replacing the collar.

The switch is not actually painted but it is darkened and may appear either black or green.  It is marked on the housing with the manufacturer "MD1" and a number that is likely a lot number or inspectors mark.  The switches are packed in sheet metal boxes, with ten switches completely assembled with fuze adapter and percussion cap.  There are also enough anchor brackets for one per switch.  The box is painted olive drab with markings in yellow.  Markings give the designation, quantity, lot number and date of manufacture of the switches.  It is also marked with the manufacturer and lot number of the percussion caps
  M.D. 1 Pressure Switch  
  Switch, No. 2, Pressure, Mk. I

The original pressure switch was designed by Millis Jefferies (Later Major-General Sir Millis Jefferies, K.B.E., M.C.) in early 1939.  His first pressure switch, originally designed for destroying railway lines, was based on a large brass casting.  It used a conventional spring-loaded striker in a partitioned barrel.  The striker was maintained in the cocked position by a steel rod that had been hardened to the point of brittleness.  The rod was secured to the closed end of the barrel with a pin.  It ran through the partitioned off section and had the spring and striker head after the partition.  A hole with a boss bearing was placed over the partitioned off section.  A plunger was dropped into the hole so that its "V" end straddled the hardened steel rod.  A safety pin fit through the boss and plunger preventing the plunger from being depressed.   While the design of the switch was good, it was too heavy to be practical.  It was redesigned to become a device weighing only 6 ounces and costing only 3/6d.

The pull switch as designed and issued by MD1 was constructed of brass except for the steel rod, striker head, and striker spring.  The barrel is cast as one piece with an enlarged section at the closed end.  The opposite end is drilled out to a depth of about 2 inches to form the barrel.  The bottom of the enlarged section is flattened and is drilled out from the bottom to form a partitioned section.  A hole is then drilled from the top for the plunger.  A small hole for the steel rod is drilled from the closed end completely through into the barrel.  The barrel is threaded on the open end to fit a screwed collar.  The base plate is a 1/8 inch rectangular piece of brass that is attached by two bolts to the flattened portion of the barrel.  There are five holes in the base plate, two countersunk holes for the bolts, two in opposite corners to allow the switch to be screwed in position, and one large one to hold the pressure plunger in transit.  The pressure plunger is turned out of brass with a stem and large pressure head.  The bottom of the stem is slotted for a short distance and has the bottom of the slot formed into a "V" shaped cutting head.  A safety pin passes through the stem.

The steel rod is assembled with the striker head and striker spring.  It is passed through the hole in the barrel assembly compressing the spring.  It is then pinned in the cocked position.  All holes into the partiioned section are then sealed with sealing wax.

  M.D. 1 Release  
  Switch, No. 3, Release, Mk. I

This switch was also designed by Lt. Col Stuart Macrae of MDI.  It was another of the successful devices that worked correctly the first time and needed no improvements throughout the war.  From design to full production took only two weeks.  About two million were made during the war at a cost of 2/3d each.

The switch resembles a small box with a hinged lid.  The main casing is constructed of sheet steel and has a lid, hinged on the long side of the box.  There are two punchouts in the body, one in the bottom that is rectangular and bent up to form a bracket for the safety pin.  The other is in the lid and has an angled side forming part of the release mechanism of the switch.  There is a hole drilled through the base to allow it to be screwed or nailed in position.  A strong leaf spring with a hammer head mounted on the free end is attached on the side of the box opposite the hinge.  An additional half length leaf spring is attached on top of the leaf spring to give a stronger spring.  The springs are retained in position by two small bolts that screw into a small rectangular plate over the end of the springs.  The hammerhead attached to the end of the longer spring has a large hole drilled through the head to correspond with a hole in the side of the case and a hole in the anchor tab.  The safety pin fits through the case, then the hammerhead, and finally the tab to retain it in the cocked position.  The opposite end of the box has a hole to accept a fuze adapter.   A short piece of spring steel with a striker mounted is riveted in position so that the striker is positioned over the hole where the percussion cap of the fuze adapter would sit.  This piece also serves to keep the fuze adapter in position after it is fitted. 

The switch is painted olive green with no other markings.  The switch may or may not be stamped with an "MD1" mark.

This is the original design of the switch and is only known as the "Release Switch".  It was used prior to official adoption as a service store.

The switch that was adopted as a service store as the No. 3 Mk. I is slightly modified from the earlier release switch.  This version has the second shorter piece of spring steel shortened and an additional wire spring fitted to give a stronger hit when released.

The switch is painted olive green with no other painted markings.  The only other mark is the British Broad arrow marking.

The use of the switches is identical.  For use, it must be cocked by drawing back on the hammer head until the safety pin can be inserted and retain it in the cocked position.  At this point the pressure placed on the safety pin makes it very difficult to remove.  A fuze adapter or spring snout is then inserted in the hole at the end of the box under the firing pin.  The fuze is placed in position and the charge attached.  An object weighing at least 1.5 lbs is then placed on the top of the lid.  This forces the lid down which forces the inclined tab on the lid to push the spring back slightly.  This action releases the safety pin allowing it to be easily removed.  When removing the safety pin, if there is any resistance, the switch is not laid properly and must be reset.  If the safety pin comes out without resistance, it is laid correctly.

  Switch, No. 4, Pull, Mk. I

This switch was designed by Experimental Station 6; a cover name for the SOE production establishment.  Originally known as "Type 6 Pull" it was designated No. 4 Mk. I when it became a service store in 1942/43.   It remained in service for a great number of years after the war, being used well into the 1980's, and may still be in service.  Switches dated as late as 1984 have been noted.

The mechanism consists of a brass body with a clip bearing two eyes attached near the bottom end.  The bottom end is internally threaded to accept a "Snout, Switch, Capped Mk. 1".  The top end has two elongated slots opposite each other and has an internal constriction formed at the bottom end of the slots.  The striker has a shoulder and firing pin formed at one end and a rounded head at the other.  A safety pin hole is drilled through the rounded head.  A "U" shaped clip is formed in such a manner that the ends conform to the rounded head of the striker.

When assembled the striker and striker spring are put in through the bottom end of the body and forced up through the constriction far enough so the head protrudes past the top end of the body.  The "U" shaped clip is put over the head of the striker and the striker allowed to move back into the body.  At this point the clip will not open enough to release the striker to pass back through the constriction.  A safety pin is pushed through the slots in the side of the body and through the hole in the striker head to ensure the device remains safe.

The body is left with the natural brass colour although a varnish is applied.  The body is marked in black with the designation, manufacturers mark, date of manufacture, and lot number. 

When the switch is set with a trip wire, a tension is put on the line to pull back on the "U" shaped clip so the safety pin will come out easily.  At this point the safety pin will be somewhere near the centre of the elongated slots.  With the safety pin out: if a pressure or pull of six to eight pounds is applied to the trip wire, it will pull the "U" shaped clip and the striker with it, out of the body.  When the clip clears the top end of the body it will release the striker to move forward under pressure of the striker spring.  The firing pin strikes the percussion cap and sets of the trap.

  Switch, No. 5, Pressure, Mk. I

This switch was designed by Experimental Station 6; a cover name for the SOE production establishment.  Originally known as "Type 6 Pressure" it was designated No. 5 Mk. I when it became a service store in 1942/43.   It remained in service for a great number of years after the war, being used well into the 1980's, and may still be in service.  It has been in British and Canadian Service and possibly with the American OSS as "Firing Device-Pressure Type A3".

The switch consists of a steel case with a hinged steel lid fitting inside the case.  A zinc alloy body is fixed inside the case by a countersunk bolt.   The end of the body is threaded to accept a " Snout, Switch, Capped Mk. 1".  There are two holes in the bottom of the case for use in securing the switch.  Two studs on the bottom of the case keep the sear springs in position.  The lid has a centre threaded hole to screw an extension into.  The striker is formed with a shoulder and firing pin on one end and a detent on the other.  The sear is formed of steel and has two studs riveted on to correspond with the studs on the case to retain the sear springs.  A safety pin passes through the case, body, and striker to ensure the device will not fire when it is in position.

An extension rod is made in two parts, a brass socket that screws into the top of the lid and a steel rod that screws into the socket.  The extension is adustable in height by about one inch by varying the amount that the rod is screwed into the socket.  If a shorter extension is required, the socket may be used without the rod.

WWII issued switches are painted dark green, postwar switches are painted tan with markings in black.  Markings give the designation, manufacturers mark, date of manufacture and lot number.

When the switch is assembled, the striker and striker spring are inserted in the end of the body with the detent facing down.  The striker is pressed back until the detent engages the sear and holds the striker in the cocked position.  The safety pin is then passed through the holes in the side of the body to make the switch safe.

The pressure necessary to operate the device varies with the position the pressure is applied on the lid.  At the farthest from the hinge pin it requires a pressure of 21 pounds, at the centre a pressure of 50 to 60 pounds is required.  When pressure is applied to the switch the lid presses down on the sear, when the sear is pressed down it disengages from the detent allowing the striker to fly forward under pressure of the striker spring.  The firing pin hits the percussion cap and fires the charge.

  Switch, No. 6, Release, Mk. I

This switch was designed by Experimental Station 6; a cover name for the SOE production establishment.  Originally known as "Type 6 Release" it was designated No. 6 Mk. I when it became a service store in 1942/43.   It remained in service with British and Canadian forces for a number of years after the war, being used well into the 1980's, and may still be in service.  Also listed as an issue item to the American OSS during WWII.

The switch consists of a zinc alloy body threaded at one end to receive the "Snout, Switch, Capped, Mk. 1".  The other end is flattened to allow it to be inserted into narrow openings.  A hole drilled in the flattened end enabled the switch to be fastened in position.  A steel lid is hinged at the threaded end an fits over the body.  The sear is hinged at the opposite end.  The striker is formed with a sholder and firing pin at one end and a detent cut on the other.  A safety pin hole is drilled through the body and another through the striker.

The switch is painted dark green with markings in black.  Markings give the designation, manufacturers mark, date of manufacture, and lot number.

When assembled, the striker and striker spring are inserted in the threaded end with the detent facing up.  The striker is pushed back until the sear engages in the detent and holds the striker.  The safetly pin must be put in place or the switch will operate.   The safety pin is the only thing holding the switch in the cocked position at this point. 

When the switch is set in position and a weight of 3.5 lbs minimum is applied to the lid, it forces down on the lid and sear it forces the striker back slightly releasing the safety pin.  The safety pin can then be easily removed without force.  A seven pound weight is the minimum weight recommended to hold the switch.


  Switch, No. 7, Pressure/Pull Electrical, Mk. I  
  Switch, No. 8, Anti Personnel, Mk. I  
  delay switch  
  Switch, No. 9, "L" Delay, Mk. I  
  delay switch  
  Switch, No. 10, Delay, Mk. I

This switch was also developed during the latter part of the Second World War.  It was also on issue to the American OSS during the war.   It is likely that the American M1 Delay Firing Devicewas designed from this switch being modified only by fitting an American Base Coupler to it instead of the spring snout.

 The switch is of the chemical delay type.  It consists of a two part cylindrical case, the upper section of copper, and the lower section plated steel.  A spring snout with percussion cap is attached at the bottom end.  In the earliest versions a brass adapter is fitted instead of the spring snout.  The lower section contains the spring-loaded striker held in the cocked position by a wire running from the striker through the top section to a retaining screw at the top.  There are two inspection holes drilled through the lower end of the bottom section.  In between the inspection holes is a rectangular hole to fit a safety strip.  The top end contains a glass ampoule containing a corrosive liquid and two pieces of cotton wicking.  The top portion of the switch is sealed by plugs and by a lead washer on the retaining screw.

When the switch is to be used, either a detonator can be fitted to the spring snout or a piece of safety fuze.  Before use the switch must be examined by looking through the inspection holes to ensure the striker is still held in position.  If not, the switch is discarded.  When laid the upper copper portion of the casing is crushed from both sides, this causes the glass ampoule to break and release the corrosive liquid.  The corrosive liquid acts on the retaining wire eventually weakening it enough to break under the pressure of the striker spring.  When the wire breaks the striker, under pressure from the striker spring is forced down onto the percussion cap.  The percussion cap fires igniting either the safety fuze or detonator.

The safety strip is colour coded according to the nominal delay of the switch.  When initially developed there were six different delay periods that were later reduced to the three listed below.  The nominal delay times for the earlier switches were timed at 77 degrees fahrenheit.  The delay strips were painted in six different colours giving delays of: black 10 min, red 19 min. White 1 hr 19 min., Green 3 hrs 10 min, yellow, 6 hrs 30 min. and blue 14 hrs 30 min.  Later issues eliminated three of the delays leaving the following: A white strip gives a delay of 1.5 hours, yellow a delay of 6 hours, and blue a delay of 16-18 hours timed at 65 degrees fahrenheit.  The delay times are effected by atmospheric temperatures. 

The switch is normally unpainted, the top is natural copper coloured, the bottom portion depending on the age of the switch may be grey in the earliest models, silver, or brass coloured.  The safety strip is painted to denote the delay times as listed above.

The switches were initially issued in tin boxes containing 5 switches of the same delay period, the tape sealing the box was the same colour as the safety strips.


  Switch, No. 11, Pressure, Mk. I 

This switch was designed during the Second World War as an electrical contact anti vehicular pressure switch.  It is designed to be laid across a road to destroy wheeled or tracked vehicles.  Because it is an electrical switch the charge can be a considerable distance from the switch if required.

It consists of two sections 2' 9" long connected by a flexible electrical wire 6" long.  Each section consists of two strips of brass held apart by insulating spacers every six inches.  The strips are contained in rubber tubing closed at the ends by rubber plugs.  One section has two additional electrical leads fitted with plugs coming out of it to connect to the battery.   Each set comes complete with a 9 volt battery.  The battery is specially designed with sockets for the leads from the switch and two screw terminals that will connect to an electrical detonator.

The tubing covering the brass contact strips is red.  This switch must be concealed or it will be easily seen on the road.  It must either be painted or covered with a thin layer of dirt or grass.  It must not be buried. 

When a vehicle passes over the switch it crushes the tubing and forces the brass strips to contact each other completing the circuit to the detonator.  It only necessary that one of the sections be run over by the vehicle to complete the circuit
  Switch, Anti Lift, No. 12 Mk. I   
  Switch, No. 13, Pull/Pressure/Release, Mk. I 

This switch was developed at MD 1 by Lt. Col. Macrae near the end of the war.  Realizing that when troops were dropped behind enemy lines or perhaps into the jungle the weight they carried became very important he attempted to do something about it.  Carrying a dozen each of the various different switches created more weight than necessary and he thought that if he could develop a universal switch that combined the three basic functions it would be easier for the troops.  The completed devices cost about 3/6d each and about 30000 were made and issued.  Unfortunately this switch did not find favour with the troops that were to use them, they were quite happy with the standard pull, pressure, and release switches.

 The body is a casting with two lugs cast on the sides.  Holes are drilled through the lugs for fixing the switch in position.  One end of the body is internally threaded to accept the spring snout and cap holder.  The other end has a round projection coming out of the top with a hole drilled in the centre to fit the pressure head.

 The switches were packed individually with a switch, 2 percussion caps, 1 extension piece, spare striker, and two woodscrews.  Ten switches were then packed in an outer tin box with a few extra cap holders.  The tin box was 6.5" X 2.75" X 3.375" and weighed 1 lb. 9 oz.

  Switch, No. 14, Charge fixing, Mk. I   
  Firing Device, Demolition, Combination L5A1