I'm trying to connect the "Hygrosens RAD-MOD"--a radar based motion detector module http://www.produktinfo.conrad.com/datenblaetter/500000-524999/502667-da-01-en-RADAR_MODUL_RAD_MOD.pdf
to the Arduino. I connected the Arduino to a 12V supply and used Vin for the radar module.
The radar module has an Open Collector output. When I connect this output with an external pullup (10K and 20K tested and working) to the Arduino (pin7) all is working. But when I connect the Radar Output directly to the Arduino and enable Arduino's internal pullup via pinMode(radarPin, INPUT_PULLUP);
there is no switching signal on the pin. Any suggestion why and how to fix it?