Differential Temperature Measurement

[sacherjj]

We have a 13'x10' room that was a screened in porch, until I enclosed it with windows.  I'm putting a drier vent in one wall, without the flapper and with finer screen, and a 4" duct fan from the ceiling into the attic.  I'll have flappers on the inside of both of those.  So Drier vent gets air from the cooler side of the house and the duct fan will push the hot air into the attic.

What I want to make is a circuit that measures inside and outside temps.  It will activate when two conditions are met:

1 - Inside air temp is above a certain level (comfort zone or 66-75 F, 18-23 C)
2 - Outside air temp is lower than inside temp.

I'm fine with a +/- 5 degrees F (3 degrees C?) on each of these measurements.  I'm wondering if I can get there with fairly cheap sensors (thermistor or similar).

Seems like if I use the same sensor, then I have a differential amp with each other and a differential for inside with a setting.

I do have a couple Maxim chips for thermocouples, but it feels like overkill for this. 

Can I get close to accuracy I want without going higher tech.  Or has anyone seen something that can be adapted to this on the cheap.  I'm seeing + $200 units for process control, when I look at differential temp controllers.

[markus_b]

I think for your requirements any simple NTC hooked up to a AD-converter will do. An Arduino with some minimal external componets (NTC, relais) will be fine.

You may want to add a LCD and some buttons to display the measured temperatures and some buttons to change the threshold and have a manual override.

I've made a thermostat for an ice-box with a AVR-Butterfly (AVR board with LCD and joystick). This has worked out well, with two NTCs and resistors to measure the temperature to about a 1/4 deg C.

[ejeffrey]

The easiest thing to use are temperature sensor ICs like the AD592 or the TMP36.  They are simple, easy to use, and can be easily interfaced to an ADC with no amplifier.  An NTC thermistor is cheaper, although not really enough to make a difference in small volume.

You could use a differencing amplifier to get the temperature difference, but it hardly seems worthwhile compared to just using two ADC channels.

[sacherjj]

Makes sense.  I've not done much with temperature, so I appreciate the feedback.  I was thinking about a 4 channel ADC, with two pots as the interface to set the two levels.  Although, I guess I could go with an encoder, button and LCD.  I mean if I'm geeking out, might as well make it cool to look at for the average folks.

[Rerouter]

for the inside temp, a NTC would work, as for the differential, probably the simplest solution would either be to add a second and use a comparator between the 2 inputs, this can either be a digital input or direct driving of the control electronics,

[sacherjj]

Reading through the datasheet, I'm liking the idea of two AD592s.  The constant current will compensate for lead length to any sensors.  Might still ADC into a Micro, but those are pretty cool looking devices.

I'll need to epoxy dip the outside sensor, I expect.