The analog inputs on Port A and Port B of the Apple III are designed to read a voltage between 0 and 2.4 volts and convert it to a number between 0 and 255. The schematic on page 82 of the Apple III Owner's Guide is drawn for a joystick with 5,000 ohm potentiometers. Only the bottom 20% of the potentiometer's range is used. Using the +12 volt power supply will result in the most stable readings.
The following circuit can be used if the entire range of the potentiometer is desired.
+12 or +5 volts
|
|
<
< Rs
<
|
| <------ must be 2.4 volts at this point.
|
<
GC input --->< Rp (1 - 2K ohms)
<
|
|
0 volts ____|
The voltage range at the GC inputs should go from 0v to 2.4v. The value of Rs can be calculated for +5 volt systems with this formula:
Rs = Rp * 2.6 / 2.4
For +12 volt systems use: Rs = Rp * 9.6 / 2.4