Temperature Comparer

I do not like paying for air conditioning. Instead, I try my best to open the windows when it's cooler outside than it is inside, and I close the windows when it's hotter outside than it is inside. Currently, the best way I have to compare the outdoor and indoor temperatures is by using a heat gun to measure the indoor temperature and checking a weather app to get the outdoor temperature. This project uses a simple circuit to notify me if the temperature outside is hotter than it is inside, thereby letting me know when to close the windows.

Project complexity

Normally, a sensor's reading is measured by putting it in series with a resistor and measuring the voltage at the midpoint. One of the issues I with thermistors though is that they are not all that accurate, requiring tuning to get accurate temperature readings. This is complicated, and I looked up ways to circumvent this. A video by Texas Instruments suggests replacing the resistors in the voltage divider with a constant current source to reduce error, but I am unfamiliar with creating constant current sources and I would have to make a current mirror to keep the current matched for the indoor and outdoor thermistor. Furthermore this doesn't "tune" the thermistors at all. Another idea that I thought of is using a resistor with one thermistor, and using a potentiometer on the other. Since I am not looking for accuracy but instead just a comparison, I could turn the potentiometer until the voltage dividers see the same voltage at room temperature.

The problem with all these solutions though is that they add complexity. Furthermore, I'm lazy, so I went with the simplest solution of using a thermistor in series with a resistor for both sides. Improving the accuracy of these sensing circuits is one way that this project can be improved in the future.

Circuit diagram and testing

I designed a dead simple circuit using NTC thermistors (see the circuit diagram to the right). I then tested this circuit on a breadboard and found that it works. The bill of materials is as follows:

-1x LM393P comparator

-1x LED

-1x 1.5kΩ resistor

-2x 10kΩ resistor

-2x 10kΩ NTC thermistor

- Power supply (5-9V)

- Wires

Perfboard and installation

With the circuit verified, I transferred the circuit onto a perfboard for a more permanent solution. I then mounted it to a wall with a pushpin and ran the outside temperature sensor through the brushes between the sliding windows to mount it outside.