Probable early 1940, the Royal Netherlands Army asked the Measurement Building to develop a detector for iron landmines. When the war broke out in May 1940, only a couple of landmine detectors were available of a model that passed the laboratory stage. That model, however, did not meet the mechanical requirements for robust military use. The principle of the detector was based on making the impedance change in a search coil (on a stick) when approaching iron objects and making that change audible using a low-frequency bridge circuit. The conventional iron mines could be detected with certainty at a distance of 75 centimetres.
Following the German invasion, the developed prototypes were used for the detection and demining of landmines laid down by the Dutch Army in Limburg and Zeeland.
After the development of the low-frequency model, work continued on the development of a high-frequency detector, even after the German invasion. The search coil is included in the ‘pendulum circuit’ of a high-frequency generator (+/- 275 kHz). There is a second, fixed high-frequency generator at 275 kHz that is connected to the first detector circuit. If the search coil comes close to a metal object, it destabilises the circuit’s frequency. The difference in frequency then becomes audible. The design is such that a very stable base frequency is achieved and that the design is as little as possible temperature dependent. The system is based on three Philips D1F pentode tubes. A lot of attention is paid to the weight: 5.5 kg on the back and 4 kg for the search reel with the handle.
The prototype device was ready in September 1940.