Added a cover sheet to separate counterweight and main PCB.

- README: describe mechanical stacking and the function of the various
  protective layers
- cvr.py: model of the cover sheet, with gnuplot and HeeksCAD output

README

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+Mechanical stacking
+-------------------
+
+From the bottom to the top, we have the following elements:
+
+- Ben case, bottom shell
+- a few drops of glue or silicone, to hold the counterweight in place
+- the counterweight, covered by protective paint
+- a few drops of glue or silicone, to keep the cover sheet in place
+- a cover sheet of thin hard plastic, e.g., the type of plastic film used to
+  make transparencies
+- isolating tape, applied to tall components of the Ben's main PCB
+- the Ben's main PCB
+
+
+Protection
+----------
+
+The counterweight is covered by one or more layers of paint, to prevent
+direct skin contact with the lead during handling. The paint may also
+offer some amount of protection against electrical contact.
+
+The counterweight is covered by a layer of hard plastic that isolates
+from electrical contact and that also resists being punctured by pointy
+components or solder joints of the main PCB.
+
+Finally, all elements on the main PCB that are unusually tall are taped
+over, to further reduce the risk of them working their way into the
+counterweight. Right now, the only component where problems are
+considered likely is the buzzer.

cvr.py

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+#!/usr/bin/python
+
+#
+# gnuplot the outline
+#
+
+def outline_gnuplot(points):
+    x0 = points[0]
+    y0 = points[1]
+    while len(points):
+	print points.pop(0), points.pop(0)
+    print x0, y0
+
+
+#
+# make a HeeksCAD sketch of the outline
+#
+
+
+def cad_line(sk, x0, y0, x1, y1):
+    cad.line(x0, y0, x1, y1)
+    cad.add(sk, cad.getlastobj())
+
+
+def outline_cad(points):
+    cad.sketch()
+    sk = cad.getlastobj()
+
+    x0 = points.pop(0)
+    y0 = points.pop(0)
+    last_x = x0
+    last_y = y0
+    while len(points):
+	x = points.pop(0)
+	y = points.pop(0)
+	cad_line(sk, last_x, last_y, x, y)
+	last_x = x
+	last_y = y
+    cad_line(sk, last_x, last_y, x0, y0)
+
+    cad.reorder(sk)
+    return sk
+
+
+def closed_outline(*args):
+    do(list(args))
+
+   
+#
+# Make the cover sheet 2 mm larger than the counterweight on all sides. We need
+# the following exceptions to avoid mechanical interference:
+#
+# - at the power connector, keep the ymin edge flush with the counterweight
+# - follow the counterweight's J-shaped space for the reset button, with the
+#   border reduced from 2 mm to 0.5 mm
+# - next to the long side of the battery, follow the counterweight's edge
+# - also follow the counterweight's bay for the battery cover's tongue and
+#   make it even 0.5 mm larger, anticipating imperfect registration
+#
+# Also, as a simplification, we don't follow steps and recesses in these cases:
+#
+# - 1 mm steps on the left and right side. Just use the larger size.
+# - the whole sponge area. Just put the cover on top of the sponge.
+# - all recesses near the batter, except the one for the lid's central tongue
+#
+# The above simplifications are possible, because, being on top of the
+# counterweight, we've already cleared the obstacles these steps and recesses
+# are designed to avoid.
+#
+
+#
+# Note: to visualize the shape defined below, plot the counterweight in 2D with
+# gnuplot and follow the shape it the mouse.
+#
+# To visualize the result, do this:
+#
+# ./cw.py >cw.gnuplot
+# ./cvr.py >cvr.gnuplot
+# gnuplot
+# gnuplot> set style data lines
+# gnuplot> plot "cw.gnuplot", "cvr.gnuplot"
+#
+
+
+def outline():
+    closed_outline(
+      # counterweight corners: (16, 46) and (15, 60)
+      13, 46,
+      # (15, 69.5)
+      13, 71.5,
+      #  (82.5, 65), (82.5, 64), (89.5, 64), (89.5, 69)
+      83, 71.5,
+      83, 64.5,  89, 64.5,  89, 71,
+      # (100, 69)
+      102, 71,
+      # (99.5, 46)
+      102, 44,
+      # (88, 46)
+      86, 44,
+      # (88, 55), (82, 50)
+      86, 50,
+      # (59.5, 55), (59.5, 56.5), (52.5, 56.5), (52.5, 55)
+      60, 50,
+      60, 57,  52, 57,  52, 50,
+      # (24, 55)
+      26, 50,
+      # (24, 46)
+      26, 46)
+
+
+if __name__ == "__main__":
+    do = outline_gnuplot
+else:
+    import HeeksPython as cad
+    do = outline_cad
+
+outline()