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Jonas posted a topic in LEGO Technic and Model TeamHi, I have just purchased a lego servomotor 88004. I tested it with a standard battery box (8881) and switch (8869) and it work well, i.e. it moved to one of the 3 positions -90, 0 or +90 degrees. It worked also without the 8869 switch when I used the internal switch on the battery box. OK. This is exactly what I need in my project, with only one exception: I need to use an external 9V power supply, not the battery box. I hoped that I could use the same arrangement like I had used with other (M, L, XL) PF motors, i.e. 9V train regulator (set up for max output voltage), extension wire 8871, PF switch 8869 and the servomotor. And ... it did not work. I was surprised because I had believed that for this simple non-proportional control no additional electronics is needed, which was proven by my initial test with the servomotor and the battery box only. I looked at Philo's excellent web pages to see the internal schematic of the battery box. I found what I was expecting, i.e. there is no electronics there. The box just outputs 4 signals (0, 9V, C1 and C2) where C1 and C2 are connected either to 0 or 9V according to the position of the switch. So the catch seems to be in the extension wire that does not transmit the C1 and C2 signals when powered from the 9V supply. Am I right? Do you know a trick how to overcome this problem, so I could control the servomotor by the 8869 switch, yet with the 9V train supply? At the moment, I see just one solution: an empty battery box used as a transducer between the external 9V supply and the switch. But it is not very nice. Anything more clever?