Recently I built a dew detector circuit. I got the circuit diagram from here:
http://www.electroschematics.com/608...etector-probe/
I coupled the output to a flashing LED circuit using a 555 timer and added a buzzer. I put the components together on a breadboard and left it running in a corner of my observatory.
Last night I was collimating my newtonian and doing some basic checks on Saturn when the buzzer sounded. It startled me (I suppose that it is its purpose anyway) so I checked the surface of a few things and then quickly closed the roof. Soon after closing the roof the detector stopped. I noticed that my telescope tube was still dry to the touch but my roof was moist.
I have not fine-tuned the settings of this circuit but I was very impressed that it performed in the way it did, i.e. detecting moisture before its a problem. Has anyone else tried something like this?