The A 1 Unit (FAB 3) (Allied designation A Mk I) was first attempt by the Germans to make an acoustically-fired mine unit. The development started in May 1940 by the firm, "Dr. Ing. Rudolf Hell" (Berlin) and was completed in September 1940. It was immediately put into operation, and the first specimen was recovered by the Allies a month later. When the first unit was in development, the following requirements were laid down:

1. The mine must fire at the approach of a ship.

2. The mine must not fire as a result of detonation of other mine in the area.

Figure 166 – A 1 Unit