Logging the environment:
 

•   Temperature    - uses a ready-made (or kit-form) PIC product. Includes serial-cable modification details.
•   Light                - a very simple PIC circuit. Optimised for low-light sensitivity.
•   Pressure           - how to connect an aneroid barometer to a PIC circuit.
•   Wireless           - turning a wireless hotspot-detector into a wifi-logger.
•   Humidity         - illustrates logging of humidity, derived from wet-and-dry temperatures.
•   Voltage
•   Frequency
•   Magnetic Field

All these low-cost PIC projects will rely on a direct TTL voltage interface into a Newton PDA, without using an RS-422 or RS-232 converter. This economical solution does, however, require an appropriate serial-cable. Some ready-made moulded serial-cables will not work unless they are modified. See the Temperature section for details of how to make a working serial-cable.
 

  It helps to have the Newton propped upright - to make a stand, see here.