Thursday 19 June 2008

W-8681 Humidity to Frequency circuit


2008 06 18 029 edited copy
Originally uploaded by bn37ej
This is guess work but it seems right (to me): The W8681 uses a humidity to frequency circuit based on the sample application given on page 3 of the HS1101LF datasheet.

The two resistors at the bottom are R22 ("514"=510k ohms) and R4 ("513" = 51 k ohms) in the sample circuit. The calibration potentometer RV1 has been omitted - the units are probably calibrated in software (which I don't know how to change).

The sonsor's capacitance increases with humidity. The 555's output frequency decreases with increasing humidity.

I don't want to mess around with the surface mounted components, so I'm going to try some resistors across the sensor to find a value that helps, and if that works fit a potentometer and resistor in series across the sensor.

W-8681 Humidity Sensor


2008 06 18b 011
Originally uploaded by bn37ej
The humidity sensor is the black finned cyclinder. A Google image search found the Humirel HS1101LF which matches visually. The datasheet is here.

W-8681 transmitter board


2008 06 18 029
Originally uploaded by bn37ej
This is the tranmitter board from the W-8681. The humidity sensor is mounted on the PCB with two 8-pin chips.

Inaccurate Humidity Readings W-8681 Weather Station


2008 06 18 005
Originally uploaded by bn37ej
The photo shows a Watson W-8681 Weather Station and two other humidity meters, after they've been in a fairly air tight room for 48 hours. The W-8681 indicates ~45 RH when the other meters indicate ~60.

The W-8681 is the second I've tried and both seem to under report RH...

This blog is going to be about correcting what I think is a fault and calibrating the humidity readings.