In order to have users select one of our special places, we need sensor input. Our first idea was to use 15 light sensors (LDRs). One for each place. But since these values might be unpredictable because of different light circumstances, we then wanted to replace these sensors with microswitches. These would cost around 1-2 euros.
Then Rene came up with a new idea, use 4 light sensors instead.

4 light sensors will be placed in a row and are constantly lighted. The ring will have 15 places where 0 to 4 holes will be drilled in it. These holes are not visible to the user. Depending on the position of the ring, the light sensors are exposed by the holes to the light. So a light sensor will either be lit up or not. So high value or low value. 0 or 1. This way we can have up to 16 combinations, enough for our installation which will feature 15 places.