The Study of Complex Mechanical Oscillators using an Inexpensive   Computer Acquisition and Processing System
  An inexpensive analog-to-digital converter has been built around the game port of the IBM personal computer. Using a single field effect transistor (FET) and three carbon resistors, it allows the game card to measure voltage, instead of its usual measurement of resistance. Since power for the FET is drawn from the game port, the total cost of components, external to the computer, is very low at about one U.S. dollar. The converter is linear enough for many applications, and is presently used to study the complex free decay of a long period physical pendulum. Software support is via QuickBasic, and the hardware/software package design was influenced by a consideration of amateur seismology needs. 
Randall D. Peters