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Gday Ian
I have designed/played with about a dozen different types of controller at present.
1) Passive manual ( just a 0-100 pot )
2) Passive manual/thermostatic ( using LM335 temp sensors )
and a dual purpose 0-100%/0-10deg offset pot
3) PICAXE controlled manual/thermostatic ( using DS18B20 sensors )
4) Simple PIC controlled
5) Really flash PIC controlled ( digital setting )
All have their own applications and costs
Also, with minor tweaks, all the above can be made for N Channel or P Channel operation.
1) Is very simple, has true linear 0-100% operation and is about the size of a matchbox ( using full sized parts ) see piccy.
5) is the most complex, as it also has humidity sensors and a LCD display, and they double the cost but arent required for basic units.
For a minimal Picaxe controlled thermostatic unit, cost of parts are in the order of $50. The P Channel is normally a few dollars more.
You also get low battery alarms, and a serial feed out if you want to watch what its doing from yr PC/Lappie.
Whilst i havent succeeded in making a truly integrated surface mount unit yet, option 3 is the easiest to work with for people without PIC burning hardware.
The circuit diag for these units is pretty simple ( as all the smarts are in the software ) and the fabrication ( other than the PCB ) is simple.
Basically, i find a box, then make a board to suit, then change the program to suit the board. It sounds complex, but is really quite simple.
You also get to program it with free software ( the programming hardware is built into the unit, so all you need is a serial cable )
Also, as you get to program it, ( and a dewheater controller is just a variable voltage supply ), you can modify it to drive peltiers, fans, motors etc.
Its all up to yr imagination ( and the current capacity of yr selected Mosfet )
Andrew
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