136. Obstacle Detector Set 42 (Stollenhorchgerät 42)

a. Purpose. The obstable detector set 42 (fig. 170 and 171) is designed for use in for-ward areas. It is used to guard and acoustically "observe" stretches of the front which are thinly held or which cannot be easily kept under full observation because of irregula-rities of terrain or poor visibility. The set enables early detection of the approach of an oppening force and, when in conjunction with remote-control mines, actively assists in the defense of the sector.

b. Employment. This set can be used in any season weather. It is particularly valuable in winter, when other types of obstacles are deprived of much of their effect, because ice and frosen ground are excellent conductors of surface ground vibrations.

c. Range and Effectiveness. One complete set can over a front up to 600 yards in warm weather, depending on the terrain. In winter conditions one set can cover a front of up to 1,100 yards.

d. Description of the Set. One obstacle detector set 42 assembled for operation consists of a control post wired to three detector circuits each of which contains from one of three acoustic sentries and a mine-detonating circuit. Each set consists of the equip-ment shown in figure 170 and listed in table II. The dimensions and weights in table II include packing for shipping. Figure 171 illustrates the obstacle detector set 42 as set up for operation, with one of three acoustic sentries employed in each detecting circuit.

Figure 170. Components of the obstace detector set 42.

Figure 171. Lay-out of the obstacle detector set 42 as set up for operation.

(1)

HT 6 acoustic sentry.

(a)

General. The HT 6 acosutic sentry (fig. 172) is a disk-shaped solid aluminum casting. It is 9 inches in diameter and 4 inches high. The slightl domed lid, which has a countersunk, hinged carrying handle, is held to the casting by eight countersunk bolts. Early models have a 1-inch diameter hole in the lid with a thin metal membrane riveted to its interior. The hole normally is closed by a threaded bakelite plug but can be opened to admit vibrations. Each sentry has four connector plugs, two for the detector circuit and two for the mine cir-cuit.

Table II. Component Parts of the Obstacle Detector Set 42.

Figure 172. HT 6 acoustic sentry.

(b)

Internal construction. The HT 6 acoustic sentry is based on an electro-magne-ticvibration system (fig. 173). It consists of a laminated-steel magnet, 2 inches long, 1 1/2 inches wide, and 1 inch thick. The pole shoes of the magnet are extended to form a square cage around two small induction coils. A small gap is left between the ends of the pole shoes. Balanced in this gap is a small vibra-tor tongue which forms the core of the induction coils. The tongue is mounted on a copper H-shaped bridge. The four setting screws at the end of the ton-gue allow fine adjustment of its movement between the poles. One end of the vibrator tongues is rigidly connected, through the vertical limb of an L-shaped carrying member, to the main vibrator. The main vibrator is a brass plate, 4 3/4 inches long, 3 1/2 inches wide, and 1 inch thick. It is mounted on a keyhole-shaped cut. The horizontal limb of the L-shaped piece is fixed to the upper pole shoe by a short piece of steel wire screwed into its end, permitting a certain amount of damped vibration.

Figure 173. Magnetic system of the HT 6 acoustic sentry.

(2)

E 131 acoustic sentry.

(a)

General. The E 131 acoustic sentry (fig. 174) is a cylindrical metal casting, 4 3/4 inches in diameter and 9 1/2 inches high. It has a wooden handle on the top. Two pairs of connecting plugs are located on top of the unit, one pair for the detector circuit and one pair for the mine circuit.

Figure 174. E 131 acoustic sentry.

(b)

Internal construciton. The E 131 acoustic sentry contains an electromagnetic-vibration system, consisting of a permanent magnet with a ring-shaped gap in which a cylindrical coil is held suspended between two leaf springs fixed to the magnet. One vibration system is formed by the coil with the leaf springs. The magnet is suspended on a U-shaped spring comprising the second vibration system.

(3)

ZV 1 selector set. The ZV 1 selector set (fig. 175) is designed to allow selective listening to serveral detector circuits as well as to set off their associated mine cir-cuits. In positions 1 to 3 of the selector switch, individual detector circuits can be engaged, while position 4 connects them in parallel. This arrangement permits liste-ning to all circuits simultaneously.

Figure 175. ZV 1 selector set.

(a)

External description. The front panel of the selector set has two circular swit-ches, the upper switch for selecting the detector cicuit and the lower switch for testing and firing the mine circuits. On the right of the panel is a circular cover to allow quick and easy replacement of the electron tube just behind it. The signal light in the center of the panel lights when the mine circuits are being tested. To the right of the control panel are three sockets for plugging in the three detector circuits, and beneath them are two permanent cables, one leading to the EVB 6 amplifier and the other to the blasting machine. These cables are stored in the compartment at the right of the case. A small case containing a spare electron tube and the signal bulb is built into the top of the cable storage compartment.

(b)

Internal construction (fig. 176). The preamplifying stage consists of an elec-tron tube (RV 12 P 2000) used as a triode. The high-tension and heater cur-rent are taken from the EVB 6 amplifier. The cathode bias is obtained by the restistance (1) bridge for alternating current by the condenser (2). The resis-tance (3) fixes the grid voltage. The mine circuit switch (15) is a single-pole switch for connecting the mine testing circuit of an indicator, a signal light (6) in parallel with a condensor (7), and three protecting resistances (8), (9), and (10). The indicator and the signal light permit the mine circuit to be checked while the protecting resistance limit the current. The condenser (7) delays the functioning of the signal light in order to permit the alternative use of the blas-ting machine with the different voltage characteristics. The resistances (11), (12), and (13) in the detector circuit selector switch make it impossible for mines to be set off when the switch is in position 4 (listening in oll all cicuits at once).

Figure 176. Wiring diagram of the ZV 1 selector set.

(4)

Cables and plugs (fig. 177).

(a)

Detector-circuit cable. The detector-cicuit cable is a heavy two-core cable. A two-partition drum carries three 165-foot lengths. A single-partiton drum car-ries an 825-foot length in the large compartment and a 20-foot lenght with a two-pin plug in the narrow compartment. Two-pin plugs and sockets with pro-tective caps are provided for this cable.

(b)

Mine-circuit cable. The mine-circuit cable is a single-core cable and is carried in a length of 1,650 feet on one drum. Three drums of cable, 1,650 feet on each, are provided. Single-pin plugs are provided for this cable.

Figure 177. Cables and plugs for the ZV 1 selector set.

(5)

EVB 6 amplifier. This amplifier unit is the same as that used in the mine-gallery de-tector set 40 (par. 136).

(6)

12 NC 28 battery. This is a nickel-cadmium type of battery with a capacity of 28 ampere-hours. It is used to operate both the EVB 6 amplifier and the ZV 1 selector set. It is used also for the mine gallery detector set 40.

(7)

Blasting machine. The standard German field blasting machine, either the 37 or 39 model, is used with this set.

e. Installing Equipment.

(1)

Control post. The equipment at the control post is set up as shown in figure 178. Setting up the control post can be done by two men. Care should always be taken to set it up on a dry surface.

Figure 178. Equipment of the obstacle detector set 42 at the control post.

(2)

Detector circuits (fig. 179).

(a)

Setting up. Three detector circuits can be connected to the ZV 1 se-lector set at the control post with the 825-foot lengths of two-core cable. Each detector circuit can contain one, two, or three acoustic sentries. The sentries are inter-connected with the 165-foot lengths of two-core cable. Both the HT 6 and the E 131 acoustic sentries have the same wiring diagram (fig. 179). The male two-pin plug H 1 of the first sentry in each circuit is connected to the control post. The female terminal H 2 in the last sentry of each circuit must always be shorted by screwing on the shipping cap which has two interconnected pins.

Figure 179. Wiring diagram for the acoustic sentries.

(b)

Testing. When all connections have been made between the control post, the detector circuit, and the acoustic sentries, a test by walking along the whole circuit should be made. The footsteps should be heard all the way. A slight fin-ger touch on the HT 6 sentry should be heard at amplification 0.5 or, on the E 131 sentry, at 1.5. In using the obstacle detector set, the oscillator circuit on the EVB 6 amplifier should be inserted. A slight singing noise in the headset can be heard when the set is switched on and also on each excitation. The heating-up period for the electron is about 30 seconds before the equipment is ready for use. A very high degree of amplification is not desirable since it tends to make background sounds more prominent, making it more difficult to recog-nize approaching sounds. By inserting the low-pass filter it is possible to cut out frequencies below 200 cycles per second.

(3)

Mine circuits (fig. 171).

(a)

Setting up. Firing of the mine circuit is always done electrically. The electric leads from the electric blasting caps are connected directly to the firing wire. The mine circuit starts from the first sentry in a detector circuit at the terminal marked M1 and ends at terminal M2 of the last sentry in the same circuit.

(b)

Testing. When all the connections have been made, and before the mine or charges and the cables have been buried, the mine circuits are tested one by one. The selector switch is turned to Prüfen (testing). If the signal light and the indicator sign fail to react on releasing the blasting machine, normal check-ing by a continuity tester should be made. Once a satisfactory test has been made, all mines, cables, and sentries should be buried and camouflaged.

Caution: Only testing personnel should remain in the area during testing.

f. Functioning.

(1)

Acoustic sentries. Both the HT 6 and the E 131 acoustic sentries receive ground vibrations through a damped harmonic system. The most receptive frequency for both sentries correspond to the wave lenghts usually experienced in soil and rock; thus a high degree of sensitivity is achieved. The damping of the system allows the recept- ion of vibrations as low as a frequency of 50 cycles per second. Both types of acoustic sentries contain a glow lamp as a high-voltage shunt to separate the listining circuit from the mine circuits. Since the resistance in the mine circuit is much smaller than that in the detector circuit, the voltage induced in the coils would be shorted through the mine circuit and would never reach the amplifier if the glow lamp were not present. At any voltage above 90 volts, however, the glow lamp lights and will lights and let the current from the blasting machine pass to the mine circuit. The resistance of the coils in the detector circuit is so great that the current passed through the circuit may be neglected and will not effect satisfac-tory firing of the electric blasting caps in the mine circuit. The colis of the sentries in one detector circuit are all connected in series. Should the leads to any one sen-try be cut, the continued functioning of those sentries on the near side of the cut is assured by connection of the two pins on the plug marked Ausgang (outlet), which is the H2 terminal.

(a)

HT 6 acoustic sentry. When the HT 6 sentry is subjected to any vibration, the vibration is taken up by the vibrator system. The vibrator tongue moving back and forth in the gab between the pole shoes causes changes of flux in the magnetic field, thus inducing a voltage in the coils. This variable voltage is brought to the main amplifier through the selector set.

(b)

E 131 acoustic sentry. Vibration causes the coil to move back and forth within the groove, inducing a voltage in it corresponding in intensity to the vibration rate. The voltage passes through the selector set to the main amplifier and can be heard through the headset.

(2)

Mine circuit. Each mine circuit is fired individually. It is not possible to fire them all at once. The blasting machine must be rewound and released, after adjusting the selector switch, for the firing of each mine circuit.