The AD 104 unit was developed for the Luftwaffe by Dr. Hell. It was started in the autum of 1942 and completed and used operationally a year later. The unit was a com-bination of the A 104 and the D 101 and was designed for use in the BM 1000 I/II mine case. The time delay in the D 101 circuit could be selected for either 4 to 6, or 6 to 8 seconds at a 20 - 30 mm suction.

The A 104 is modified to include the D 101 and is associated relay R4 and R5.

It is further modified by the addition of W 10 which changes its normal anti-counter-mining action. Addition of W10 reduces the resistance of the charging circuit from B2 to C4. There will therefore be no discharging of C4 and C5 if the sound does not rise rapidly from initiating level to firing level.

Figure 239 – AD 104a Unit Switch