Fan controlled by humidity sensor

I have a problem with machine tools getting condensation on them when conditions in the shed read dew point.

A possible solution being explored is to circulate air with a fan, possible inducting outside air, when humidity approaches condensing conditions.

A quick search reveals the HDS10 resistive humidity sensor for a dollar or so on eBay.

Above is the HDS10 humidity sensor.

Most low cost humidity sensors use a humidity dependent capacitive element, the HDS10 is different in being a humidity dependent capacitive element and is therefore simpler to use with microcontrollers with ADC input.


The above graph is from the datasheet. It is intended primarily for sensing high humidity (dew point, condensing conditions) which suits this application.

It seems that the sensor must not be exposed to more than 0.8V.


Above is the manufacturer’s suggested circuit.

This sensor can be integrated into my Generic heating / cooling controller design (hcctl). hcctl comprises flexible firmware that is tailored by EEPROM calibration data and options.

Clip 207Above is a calculation of the ADC count in hcctl. Of concern is that the equivalent source resistance Req is well above the recommended ~10kΩ for the ATTiny25 MCU used (it degrades ADC conversion accuracy).

Clip 208Above is the same calculations for a revised voltage divider of 82kΩ and 15kΩ. It achieves the target Req range and has higher ADC count sensitivity… win/win.

Screenshot - 23_07_16 , 11_11_33

Above is the EEPROM contents for setpoint=70%, differential=10%, alarm=90%, and minimum run time of 1hr.

Now to wait from some parts on a slow boat from China.