Sensing the capacitance value of a material - for an ID system
There's no need to have any kind of conductive connection between Base and puck. It actually forms a condenser together with base contacts. There is one condenser formed for each of 8 surfaces on pad and the circle in the middle is common contact for all of the capacitors.

Different puck types have different number of surfaces, from 1 to 8. Minimum is one because base has to recognize that puck is present and if puck does not have any surfaces it will be the same as if it is not placed on base.

If puck cannot be rotated on base (there's a notch in the picture) then 8 surfaces correspond to 8 bits, which gives 255 different combinations (excluding 0). If rotation by 90 degrees is possible then you have 4 bits for different pucks.



The surfaces can be detected by measuring capacitance of corresponding capacitor. This could be done with very good accuracy. You just need to properly set capacitance threshold.