On 7/14/2013 10:52 AM, Phil Keller wrote:
> A friend of mine was wondering if I could build a simple device for
> him.  He is looking for something that senses the outside and inside air
> temperatures.  If the outside temperature is greater than ~70 and if the
> inside temperature is 5 degrees greater than the outside temperature it
> would enable an exhaust fan.  (We would probably need some hysteresis on
> the temperatures to keep the fan from oscillating ON-OFF.)
>
This is one of those situations where I wouldn't use semiconductors. A=20
household thermostat for heat and air will do what you want and only=20
require the addition of a power supply and a relay.  An older mechanical=20
type with the thermal metal and mercury switch wouldn't even consume any=20
power until needed.

Set the thermostat at 75F and if the inside is greater  then the relay=20
triggers. The outside would have to be over 70F for the inside to be=20
warmer to enable the exhaust fan.

That system for exhaust fans was/is actually used in thousands of=20
business that control the temp in hog/chicken/turkey production houses.

Mark

--=20
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