Thanks redbeard I may have to quiz you on a few things :-)
I'm still leaning towards the ATMEGA88/168 to keep cost low. I'm doing a lot but the reality is I need to split the project across a few microcontrollers anyway for modularity and to get around that lovely "feature" of serial communications that only one piece of software may access a comms port at any given time. This means Focuser will be on it's own micro freeing up a lot of spare code.
The DHT22 looks like a much better option than the DHT11. I'll look into that. The biggest gripe I had with the DHT11 is its inability to get below 0 degC, a situation which many people may find themselves in.
I haven't decided on a MOSFET yet but I have buckets of them at home so I'll pick any suitable one. On a slow switching circuit like a dew heater the critical part is a low Rds_on value which limits you to any one of about 10000 different ones
. As switching frequency increases then the rise/fall time become critical to reducing power loss as well, but this won't be the case here.
I actually do have a question not so much on the "how" but rather the "what". Coding up some algorithm is easy when you know what you want to do.
With temperature controlled dew straps what is the control scheme you're aiming for? Are you trying to keep the tube at a certain temperature compared to ambient? Are you trying to keep the dew strap at a certain temperature? How do you compensate for the location of the temperature sensor (i.e. the tube is nice and warm but the middle of your 12" corrector is dewing up because the heat isn't getting all the way to it.
This uncertainty and the assumption that all devices will dew up equally lead me to the theory that you bias the dew straps as a percentage and then control them globally based on the results of a single temperature / humidity measurement. i.e. when the humidity goes above 95% then put all the dew heaters at their certain biased starting positions.
Also which temperature did you use at the dew strap? A Dallas 1-wire sensor as well?
Cheers,