more simple and less mechanical...
Simple Baloon Release Mechanism
An idea for a more simple, less mechanical, possibly more reliable release mechanism. A baloon (long skinny as used for making baloon animals or hats) in a clear glass or plastic tube, open to the atmosphere, a valve in the inflating end to pressurize, the baloon painted black at the other end to block light from an LED to a light sensor. When the baloon expands linearly along the tube to block the light path through or just outside the tube, it would cause an electro-magnetic latch to release the package. It would take some experimenting and calibration to find the proper size tube and baloon, and amount of pressure to make it work reliably... but, certainly more simple and less mechanical, possibly smaller and lighter, too.