Many answers already, but:

Puematic closed system where pressure is increased by user standing on 
rock. Then detect pressure rise with anything from a sprinf loaded 
diaphragm and a microswitch to a 'pressure sensor'. Complete electrics 
can be housed in sealed unit.

        Russell


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist