need temperature based circuit

Hey folks,

I'm trying to create a very simple circuit (such as an OR circuit) to compare ambient air temp to air temp coming from an HVAC vent. If the temp from the vent is colder or same as ambient, the circuit stays open or sends a negative or whatever else. If the air from the vent is hotter than ambient temp, it closes the circuit or sends a positive, etc.

I had been thinking of just a thermostat, but that won't compare to ambient but to a temp setting. I have no experience with thermistors and am not sure if they would fit the application. From what I know of them, it would be similar to comparing to a set temp.

Any and all help appreciated. Like most of us, price and ease of building are concerns.

Thanks.

-------------
Webpager - Upfitter, computer guy, car guy. I can't tell the difference between Columbian or African coffee, but I knew at age 4 the difference between volts, watts and ohms.

Google temperature comparator circuits; images may yield the best results.

The simplest would use Comparators - eg, LM339 quad comparator, single supply.

Most of what I see is comparing to a set temperature.

Would I use thermistors to compare resistance based on temp? Thermistor #1 for ambient, #2 for HVAC vent, where if higher resistance on #1 (lower temp?) would indicate hotter air coming from vent.

I'm pretty good at searching. If anyone has some other terms or maybe links, again, I'd greatly appreciate it.

-------------
Webpager - Upfitter, computer guy, car guy. I can't tell the difference between Columbian or African coffee, but I knew at age 4 the difference between volts, watts and ohms.

You substitute the set temp for the reference temp.

One comparator should do it. The same thermistor or whatever temperature variable resistance in both comparator inputs (+ & -) with the same resistors to the rail (+V or GND).

Comparator outputs are active (often LOW = GND) when V+ > V-.

If the logic or "sense" is wrong, simply swap the + & - (V+ & V-) inputs.

Can you suggest specifics? I am not very versed with the IC side.

-------------
Webpager - Upfitter, computer guy, car guy. I can't tell the difference between Columbian or African coffee, but I knew at age 4 the difference between volts, watts and ohms.

What did google return?

maybe something like this: https://www.seekic.com/circuit_diagram/Sensor_Circuit/TEMPERATURE_DIFFERENTIAL_DETECTOR.html#

Yes, provided the thermistors don't go too low in resistance, hence 2 low series resistors across the supply.
Usually such circuits swap R6 the R3 thermistor... Hence R5 & R6 limit the max current thru the thermistors.
It's the same theory - two voltage dividers consisting of variable resistances, one divider to the non-inverting (+) and the other to the inverting (-) inputs, whose voltages are compared.   

Comparators are a special case of an OpAmp, hence OpAmps can be used.


Your linked circuit includes a timer function which may not be necessary.   
And of course its controlling an AC fan.

just trying to help the OP out. the circuitry on the output (pin 3) would be changed.

I'm thinking this project is beyond my capabilities. I had thought it might be accomplished without an IC using one or two input devices and a relay or two to switch and output.

-------------
Webpager - Upfitter, computer guy, car guy. I can't tell the difference between Columbian or African coffee, but I knew at age 4 the difference between volts, watts and ohms.

Sorry Ween - I thought it was webpager that posted that circuit.

webpager - no, it can;t be done with plain relays because you are comparing two resistances - ie, voltage levels; specifically when one is higher (or lower) than the other.
But the circuitry should be simple - 2 resistors & thermistors as + & - inputs, 1/2 a LM393 dual or 1/4 of a quad LM339 comparator, and a MOSFET output (with 2 resistors) to turn on the fan or whatever load.

It can also be done with CPUs etc - eg, PICAXE 08M2 or AT-tiny chips. Someone on mp3car supplies cheap ATtiny (or whatever they're called) PCBs...

There may be commercially available units - you'll just have to search - but they'll probably be (more) expensive.