already added a litle relay inside the unit that is activated by the reed switch.
i think i should try change the pot resistance value or something but i already try it unsuccessfully..
any help?
This might be slightly difficult to explain. I might have to draw a schematic.
The pot is the easiest way to utilize your reed switch.
1. Figure out which direction the voltage moves to turn the temp up/down.
2. Bugger that voltage in the proper direction with the reed switch; one end of the reed switch connected to the wiper of the pot, and the other to ground (or positive rail, depending on the logic/direction) with a series resistor.
So when the reed switch closes, the voltage at the wiper is skewed down/up by the pulldown/up resistor.
If you had room in there, you could use another potentiometer to "skew" the voltage. There are many ways to do that. You could either use it like a variable resistor and wire it just as previously described, so the amount of temp drop will be adustable. Or you could use a relatively stiffer pot (lower resistance) than the original and wire the wipers in parallel. Then you can use the pot to change the temp up or down, so you don't even have to figure it out which direction does what until after you close the thing up. What you would really want to do in this case is cut the trace off the original adjustment pot wiper and bridge it back with a resistor. Then the reed switch will connect the wiper of your auxiliary pot to the original trace, bypassing this resistor you added. So your auxiliary voltage will largely override the original signal when the reed switch is closed.
This is all operating on the assumption that the original pot is producing a variable voltage between DC rails, which the value of this voltage is what is ultimately controlling the temp.
But it is also possible that the pot is used in other ways, as part of a ladder, as a variable resistor. In which case you have to figure that out and adjust your game plan. Figuring out what the rails are and seeing if the pot is attached to both power and ground or not would be a good starting point.