Taken from selman "555 Ramp Generator"
The 500pF is a capacitive humidity sensor,
e.g.
http://www.meas-spec.com/downloads/HS1101LF.pdf
The 100k is a thermistor, e.g.
http://www.murata.com/products/catalog/pdf/r44e.pdf
The Discharge output is an open collector and can pull down a pin on a a micro-controller (MCU) with a weak pull up e.g. 3V3 or 5V over a fair distance with good nose immunity.
Many MCU's have capture hardware that can record a free running timer during a rising or falling edge event, a very precise measurement.
The capacitor can be calculated using the fixed 500k Ohm and time measured while the pulse is high, and thus the relative humidity found (see the datasheet).
The thermistor can be calculated based on the measured capacitor and the time measured while pulse is low, so it is a less accurate measurement but none the less useful.
