The DM 1 unit is a combination pressure magnetic unit designed by Hasag, Leipzig, for SVK, for use with the LM and TM mines. Work on it began early in 1942, and it was com-pleted and ready for operational use in the summer of the same year. The first unit of this type was recoveres in Normandy in July 1944, Figure 248.

This unit utilizies the M 1 with a sensitivity of 5 mg and the D 1 with a delay of 8 se-conds at 15 to 25 mm suction.

A thermal delay switch (15 to 40 seconds) determines the interval after actuation of M 1, during which actuation of D 1 may fire the mine.

Figure 247 – LMB Tail with D 1 Unit

Figure 248 – DM 1 Unit

Arming. When the hydrostatic clock runs off its delay setting, A-G and B-C closes. B then charges C through W3 and the closed contact (4-5) of R1. When C is charged, R2 operates, breaking 4-5 and making 4-3, thereby removing the detonator from the circuit. Eighteen minutes later, E-F closes, the magnetic component goes through A.L.A. and is armed.

Magnetic-Actuation. Actuation of the magnetic component closes the needle switch and operates the air core relay, thereby closing S5 to contact No. 1. Hold-on current and thermal delay-switch heater current pass through the normally-closed contact of the thermal delay switch. This condition persists until the thermal delay switch opens, breaking the magnetic hold-on and heater current. The magnetic component then re-turns to normal.

Pressure Actuation. A decrease in hydrostatic pressure closes pressure switch P. This causes B to energize R1, breaking 4-5 and making 4-3. Battery current to C is thereby cut off, causing C to discharge through W3 and R2. When C has discharged suf-ficiently, it no longer holds 4-5 open. The foregoing constitues a single pressure actuat-ion for which nine seconds continuous closure of P is required.

Combination Actuation. If magneitc actuation occurs first, pressure actuation may be completed at any time within 25 seonds thereafter. At the end of the 25-second pe-riod, the thermal delay switch breaks the magnetic circuit. Pressure actuation within the 25-second interval causes contact 4-5 of R2 to make, firing the detonator through con-tact No. 1 of S5.

If pressure actuation occurs first, P must be held closed until the magnetic actuation is complete, if the detonator is to fire. Otherwise, opening P de-energizes R1, recharging C and thereby opening 4-5, which takes the detonator out of the firing circuit.

Figure 245 – DM 1 Unit circuit