The device is for the function testing of a signal lamp monitor controlled by the feed current from a signal lamp and formed as a threshold value switch. The upper and lower switch thresholds are determined by the highest and lowest signal lamp feed currents. In order not to use special control conduits for the function testing process, the threshold value switch with the signal lamp (L) switched on is adjusted to different switch thresholds (OS1,US1,OS2,US2). These switch thresholds with the signal lamp driven by daytime voltage (30 volt) represent the current window (NF) for signal lamp monitoring during nighttime operation and when the lamp is driven by nighttime voltage (20 volt) they represent the current window (TF) for signal lamp monitoring during daytime operation.
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