Have you taken a look at the datasheet for these sensors? www.fotek.com.tw/pdf/etc_221.pdf
Page 7 shows what I mentioned earlier. That the max distance is based on the material you are using to trigger it. The max distance of one is 4mm, but that is based on 1mm thick steel equal width to your detecting distance (probe 4mm then steel needs to be 1mm tall and 4x4mm wide for max distance) Other materials are less sensitive/distance detection.
Steel is the max 1.0 x 4mm
Chrome and nickle are 0.9 x 4mm
Brass is 0.5 x 4mm
Aluminum and copper are only 0.4 x 4mm
Interesting since you may have steel or chrome screws/hardware under the copper which may give a false reading before the copper is in range. The chrome hardened rods may trigger it before aluminum or copper sitting on top of the bed? I don't thinks so with 4mm range, but a better sensor capable of 7-10mm distance might.
Haven't had a chance to work with mine yet, but they are going to be very interesting to experiment with.
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