Plus they produce a lot of noise. Originally I planned to locate them at the upper corners of the mattress, but I had to move them directly under the sleeper (fortunately they are not really noticable). Furthermore detecting motion was harder than I thought: you must have a high sample frequency to not miss any spikes and you cannot simply take averages (which would help to clear the noise) because on every "up" movement there follows a "down" movement, which basically negate each other. So you have to track differences and add up their absolute values. In the afterthought that's absolutely clear, I just had not thought of that before;-)
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