58. General

As early December 1943, the Russians had developed a frequency-induction fuze desig-ned to detonate mines or charges when an electronic mine detector was swept over the fuze. In January 1944, the Germans experimented with a number of captured Soviet fre-quency-induction fuzes and found that 86 percent of them were unreliable. The Germans perfected this type of the fuze early 1944 and produced the SM-12 fuze which was much superior to the Russian model. A modified and simplified model was produced by the Germans at the end of World War II. None of these fuzes were known to have been ac-tually employed in combat.

59. Frequency-Induction Fuze SM-12 (Frequenzinduktionszünder SM-12; F.I.Z. SM-12)

a. Description. The frequency-induction fuze SM-12 (fig. 61) is an instantaneous, electri-cal fuze in which frequency circuit. Two models were made. The earlier model is housed in a black, cylindrical, laminated-wood case and the later model is housed in a black, cy-lindrical, bakelite case. Both models measure 6 3/4 inches in diameter and 3 3/4 inches in height. They weigh about 2.5 pounds. The contents of the two models are –

Item

Sensitive relay

Dry-disk rectifier

Pick-up coil

Condenser

Resister

Leaf type arming-delay switch

Electrolytic-delay arming switch

1.5-volt dry-cell battery

The internal elements of the fuze are mounted on sponge-rubber pads and are bolted to the case. A tube for the detonator cable passes through the fuze and out at both the top and the bottom of the fuze. It is closed by a cork on the top and by a shipping cap on the bottom. An arming nut or screw is located on the top of the case, with a white arrow to indicate the direction to turn it for arming the fuze. A cable connects the fuze to an electric detonator (fig. 62).

b. Employment. This fuze is designed to explode a mine or charge to which it is connec-ted, by picking up the signal emitted by an electronic mine detector sweeping mine fields and roads.

Figure 61. Frequency-induction fuze SM-12.

c. Functioning. When the search coil of a frequency-bridge type mine detector, operat-ing in the frequency range of between 800 and 2000 cycles, is passed over an armed fuze within a maxium distance of 17 inches, its signal is picked up by the pickup coil in the fuze. This closes the secondary or safety arming switch, completing the circuit of the internal functioning of both models are as follows:

(1)

Wood-cased model. See the wiring diagram (fig. 63). The circuit is made up of three stages: the arming circuit, the pickup or receiver circuit, and the firing cir-cuit. However, certain components are included in more than one of these circuits.

Figure 62. Internal view of frequency-induction fuze, plastic model.

(a)

The arming circuit comprises the initial and the delay arming switches, dry cell 1, and relay 3. Relay 3 is normally closed. When the external arming screw is unscrewed the delay arming switch No. 2 is closed and the circuit from dry cell No. 1 through relay No. 3 is closed opening the firing circuit. The firing circuit will remain open until cell 1 has discharged to such an extent that it can no longer hold relay 3 open. This should occur fairly rapidly since the coil of relay 3 is short-circuited by resistance 2, a short spiral of wire of 2.5 ohms resistan-ce. The reclosing of relay 3 completes the arming of the fuze.

(b)

The receiver circuit comprises the pickup coil (which is tuned by two fixed 0.01- farad condensers), the dry-disk rectifier, and the coil of re'ay 1 (the microammeter relay). A signal picked up by the coil is rect' fied. If sufficiently strong, it causes relay 1 to close. This completes the circuit (cell 2 through the contacts of relay 3, the coil of relay 2, contacts of relay 1, initial arming switch) and causes relay 2 to close. At the same time, since the coil of relay 1 is in parallel with that of relay 2, an additional current, limited by the resistan-ce R1, will pass through the coil of relay 1 and tend to keep its contacts clo-sed. The closing of relay 2 also completes the firing circuit (the contacts of re-lays 2 and 3, the initial arming switch, and cell 2) and fires the electric detona-tor. The pickup coil has an inductance of 750 microhenries and a restistance of about 1000 ohms. The resonant peak of the tuned circuit occurs at about 1050 cycles per second. The microammeter relay (relay 1) closes at a current of about 10 microampers.

Figure 63. Wiring diagramm of the two models of the SM-12 frequency-induction fuze.

(2)

Plastic-cased model. The internal components and layout of this later model are generally similar to those of the earlier model, with the exception that the leaf type arming-delay switch has been replaced by an electrolytic-delay switch which elimi-nates the firing relay. The circuit is much simpler than that of the earlier model. The arming-delay device holds the firing circuit open, after the arming nut has been un-screwed, for about 2 hours. The device consists of a small iron case partially filled with liquid, with a rubber diaphragm over the otherwise open end of the container. A central metal boss protrudes from the top of the diaphragm amd continues under-neath as a screw, ending in a sharp point which rests in a recess in the top of an ebonite post attached to the base of the case. Above the case and contacting the boss (when in the unarmed position) is a metal leaf spring supported between two copper strips. Above this leaf spring is a contact post. When current passes through the device, electrolysis takes place, and the gases released force up the rubber diaphragm and cause the leaf spring to snap up, striking the contact post and completing the arming circuit of the fuze.

d. Installing and Arming. Turn the arming nut in the top of the case in the direction of the arrow. This action closes the initial arming switch (leaf type) and completes the cir-cuit to the delay switch, with becomes armed after a delay of 1 1/2 to 2 hours.

e. Neutralizing.

(1)

Method 1.

(a)

Remove the cover leading to the mine or charge. Without shaking ot jarring the fuze, carefully pull out the cable with the detonator attached.

(b)

Bury the end of the cable, with the detonator attached, in the ground and them cut the cable. This acts as a safeguard in case the detonator explodes.

(2)

Method 2. Unscrew the fuze from the mine, if the fuze is screwed to the mine by the flash tube, and pull the cable and detonator out of the mine.

(3)

Method 3. Neutralization may also be accomploshed by pushing a wood plug through the arming-screw hole and forcing open the arming switch.

(4)

Precautions.

(a)

Recent development may well have made this type fuze impossible to neutralize either by cutting the cable to the detonator of by forcing the initial arming switch open, without such action causing detonation of the mine or charge.

(b)

A battery may have been connected in series to the cable wires leading from the fuze to the detonator. If so, these wires must be cut one at a time; other-wise the mine will explode when the two cable wires are shorted by the cutter.

(c)

The arming contacts may be activated in such a way that insertion of the stick into the arming hole closes the contacts and sets off the mine or charge. Al-through in the majority of cases it may be safe to neutralize the fuze by in-serting a stick or other nonconducting material into the arming hole and open-ing the contacts, it is best to assume that all these fuzes are unsafe to neu-tralize in this manner.

f. Detection.

(1)

The SCR-625 electronic mine detector may be modified so that it can safely be used to locate the SM-12 fuze. Modification is accomplished by reducing the power of the SCR-625 so it will not actuate the detonator. Power can be reduced by plac-ing a 2500-ohm resistor across terminals 4 and 5 of the 1G6G oscillator tube. See TM 11-1122 for detailed instructions.

(2)

When the SCR-625 is operating on reduced power, it is still able to locate either the SM-12 or standard metal mines from a distance of 2 to 3 feet.