- Thermistor, 38k Ohm, Beta=3970
- 44k Ohm Resistor
Time to Complete:
This is the first project I have done that has a practical application. The purpose of this project is to make a data logging thermometer. That means, I will be able to detach the Arduino and leave it on 9V battery power while it takes temperature at specified timing intervals. Later, I can reattach it via USB and read off the temperatures. As you can see from the picture above, the electronics is very simple.
The temperature senor (thermistor) is merely a resistor that is very sensitive to small fluctuations in temperature. The thermistor is characterized by its resistance at a certain temperature and a value beta that represents the exponential decay constant in converting from the resistance of the thermistor to the ambient temperature. The thermistor should be paired with a resistor of the same resistance. Note that mine are a little mismatched. This and the fact that the thermistor I had did not have a data sheet (Who knows if the value I looked up was correct?) means that I the values from the logger will probably be off a little bit.
The program for this project is the most complicated one I have seen from this book so far. The Arduino is programmed to accept commands through the serial monitor. The units of the logger can be set to either Celsius or Fahrenheit. The interval between data points can be changed, and the data logging can be stopped, started, or read out. The measured resistance of the thermistor is converted to the appropriate temperature and stored in the local memory of the Arduino. This memory is kept even if the Arduino is unpowered. Therefore, it must be manually set between data sets. Instead of a video, I have included a graph of the temperature of my living room below. The thermometer I have in the living room said 73.5 F at the time the reading. The Arduino logger did ok. An accurate beta value and a well-matched resistor would probably fix the accuracy problem. It was definitely not 76 F in my living room.