Electrically, the joypad emulates two potentiometers with a common wiper (see Figure #1), and that is precisely the symbol we have adopted for it. The simplist method for using the pad involves placing two sets of diodes across the orthogonal axes and impressing an alternating voltage across them, picking off X and Y information on each half cycle. The method outlined in Figure #1 is slightly more complex, using two analog gates rather than diodes to switch each orthogonal axis on and off. It has, however, the advantage of using only a single ended D.C. supply. To explain the operation of the circuit, a two phase D. C. clock (Note: It is not necessary that the pulse widths of the clock be equal) is used to switch a reference supply "V" alternately across the X and Y axis of the pad, simultaneously switching the wiper into X and Y detection circuits. The output X and Y pulses have amplitudes as a function of the wiper position. These pulses may be smoothed and used as control voltages (See Figure #2) or by properly selecting the pulse widths, the pulses can be used directly to control some external circuitry or device such as a T.V. game, computer cursor or machine tool.
In conclusion, there is now available to the designer a simple, inexpensive input device, rugged enough to withstand the abuse of consumer applications. The door is now open to applications in interactive graphics, character recognition and general control.
1 (ELLIS 1) T. O. Ellis and W. L. Sibley, "On the Development of Equitable Graphic 1/0", IEEE Transactions on Human Factors in Electronics, Vol. HFE-8, No. 1, March 1967.
Bibliography
Fletcher, W. E., "ON-Line Input of Graphical Data", presented at the 1963 Digital Equipment Users Society Meeting, Livermore, California.
Note Lisa's control button.
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