Add Magnetic card emulation for C-Netz

Emulation can be done with a coil connected to sound card.

Alternatively an Attiny85 can be used to control a coil.

cad/coil.scad

@@ -0,0 +1,18 @@
+$fn = 100;
+durchmesser=30;
+rille=8;
+wand=1;
+
+module torus(d1, d2) {
+    rotate_extrude(convexity = 10)
+    translate([d2/2+d1/2, 0, 0])
+    circle(r = d1/2);
+}
+
+intersection() {
+    difference() {
+        torus(rille+wand*2,durchmesser-wand*2);
+        torus(rille,durchmesser);
+    }
+    translate([0,0,-5]) cylinder(h=10, r=(durchmesser+rille)/2);
+}