Luftwaffe Forelle Unit

Unit Construction. The circuit of the unit is shown in figure 261. Its optical construc-tion was similar to that of SVK test model 4, except that a cylindrical shaft with several transverse walls was fitted to shade the unit from the direct rays of the sun. For opera-tional use the unit was placed in a log with an explosive charge placed about 25 feet be-low, and floated downstream. The circuit is so designed that the unit will fire only after three looks; however, by removing the thermal switches, the required number of looks may be reduced down to one.

Unit Operation. The operation of the Forelle unit is as follows. While switch E is open, contact arm (1) of relay I moves between its mid-position and position 2 until the sele-nium cell S is illuminated, in which case (1) moves to position 2. After a preset time, switch E is closed by a delay clock, permitting current to flow from the battery through I and opposing the current flowing from S. Resistors R1 and R2 are calibrated to that a light intensity of over 300 lux incident on cell S creates sufficient current to overcome the battery-bucking current. When S is darkened by passage under a bridge (the same rsult would occur due to darkening by evening twilight or other shadows) the battery current overrides the S-current closing contact (1) to position 3, permitting current to flow through thermal switch T1, which closes after a ten-second delay. After the 10 se-conds, the unit should be clear of the bridge shadow, permitting the S current to over-ride the battery current and throw contact (1) back to position 2. Current will then flow through T2, closing contact T2 and opening the circuit for another look. When a second look occurs, the same sequence follows through T3 and T4, closing contact T4. The third look puts the battery across the detonator and the mine fires.

Designated units in this category were the S 102, S 103, S 104, and S 105. The S 102 was a passive optical unit designed for use in the Wasserballoon aganist bridges. It was started in the Fall of 1944 and completed that winter, and was used operationally. The S 103 was similar to the S 102, a passive optical with six photo-electric cells. This was started the same time as the S 102, but never completed. The S 104 and S 105 were active acoustic units with transmitter and receiver and a light-flashing component of 13 flashes per second.

Figure 261 – Forelle Unit Circuit