DC alternative; was humidistat

[Richard Brekne]

Here is a unit that might do the trick... but you'd have to have two of 
them, one for each direction. Alternatively you could have just one to 
drive a relay that would turn on and off a a humidifier/de-humidifier 
pair. But that would leave you with a single RH level to switch on. A 
pair would let you switch on and off each controller at independant RH 
levels. 

http://www.tamtech.com/humitrak.htm

These are somewhat less expensive then those David came across.  Between 
70 and 90 dollars apiece... depending on which ones you select.

Cheers
RicB