The KoalaPad, designed by Dr. David Thornburg, was immensely popular in schools and the home due to its low price and ease of use. While other drawing tablets used an intricate grid of wires to read the pens position on the tablet, and relied on special controller cards and software, the KoalaPad simulated a joystick and even used the Apple II's joystick connector. This made the tablet very inexpensive and workable with any software that used the joystick.
The pen used by the KoalaPad is just a stick of plastic. A finger or any other material can be just as effective.
The tablet is controlled by a very simple circuit with little more than just one IC and a couple of resistors.
The KoalaPad can be tested as any joystick can. In the upper left corner, the X,Y output should be aproximately 0,0, in the lower right corner 255,255, and in the center about 130,130. The pads become less accurate with age.
The tablet itself, it appears, was made by AMP.