Here's my dew controller.. using the 555-based PWM, the right-hand switch is for the fan on my GSO dob, the left-hand switch controls the dew heater which has two outlets, I have two eyepiece heaters from
www.dew-not.com.. The controller itself cost me just over $20 in parts including the case, knob, switches etc.. I will be using a 7ah 12v gel battery.
http://www.zitafarm.com/astro/dew.jpg