7555 Dual Sensor Circuit with Pulse Output

7555 type voltage controlled oscillator used for frequency output from a capacitive moisture sensor and thermistor. The 180pF is a capacitive humidity sensor, e.g. http://www.meas-spec.com/downloads/HS1101LF.pdf The 470k is a thermistor, e.g. http://www.murata.com/products/catalog/pdf/r44e.pdf The 7555 is like a NE555 but much better, for example it should work at 5V where this sim will not without the logic gates. http://www.intersil.com/content/dam/Intersil/documents/fn28/fn2867.pdf The 7555 Discharge output is an open drain 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 1.5Meg Ohm and time measured while the pulse is low, 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 high, so it is a less accurate measurement but none the less useful. Simplified view of circuit board: http://epccs.org/indexes/Board/MSPM01/

published 10 years ago