Nuts and Volts - November 2014

resistance progressively drops as more light is added (the hand is raised), bringing it down to about 2900 Ω when the light sensor is uncovered. This doesn’t really tell us anything about the absolute value of that light as measured in some accepted dimension such as lux, but it does tell us that we are able to get a relative change in voltage as a function of changes in light level due to moving our hand. We will use these facts in our labs.

Sensing Temperature With a Thermistor IC

A thermistor is a resistor whose resistance varies with temperature. This is similar to the light sensor that varies resistance with light levels. The Arduino 101 Projects Kit has a MCP9700A thermistor IC (Integrated Circuit) that contains signal-conditioning circuitry that outputs a voltage proportional to the change in temperature. This circuitry simplifies our task since we can use the Arduino ADC to measure the voltage and directly translate the reading to a temperature.

The IC uses a T0-92 package as shown in Figure 4. Figure 5 shows a drawing from the datasheet that illustrates the pin locations. Figure 6 has the breadboard and schematic symbols we will use. This device measures in Celsius, and its accuracy is ± 2°C from - 40°C to 125°C. For those of us in America, we translate Celsius to Fahrenheit in our lab software using the following formula: °F = °C x 9/5 + 32.

The MCP9700A outputs 10.0 mV/°C ( 10.0 millivolts [.01 volts] per degree Celsius) scaled for 500 mV output at 0°C. Thus, if we measure 500 mV (0.5V) on the ADC, we know the sensor is detecting 0°C. We can use this base point (500 mV for 0°C) to calculate the voltage for the maximum accurate temperature of 125°C, and the minimum accurate temperature of - 40°C.

Stop for a moment and think about how you would do this. First, look at the maximum accurate temperature: 125°C. We know that we have 500 mV for 0°C and that the sensor outputs 10 mV per °C. This means we have 10 mV per °C for 125°C, so we will see an additional 10*125 = 1250 mV above the mV level for 0°C.

We add the mV at 0°C/500 mV to the 1,250 mV for 125°C, and calculate that we would read 1,750 mV as the voltage for our maximum accurate temperature reading. If this isn’t clear, then please write it out and do the calculations by hand so you understand how to convert from volts to temperature.

Let’s repeat this process to determine the voltage for the minimum accurate temperature, which is - 40°C. See if you can do this before reading further. We have a total of - 40°C, which would equal a -400 mV change. Since we know that we would read 500 mV for 0°C, then we can subtract the 400 mV to get a - 40°C reading of 500 - 400 = 100 mV. We now know that if we read 100 mV, we have - 40°C; if we read 500 mV, we have 0°C; and if we read 1750 mV, we have 125°C. Got it?

We saw in Chapters 6 and 7 that our ADC looks at voltages from zero to five volts, and divides that into 1024 steps with 0 being zero volts and 1023 being five volts. We further saw that each ADC step represents 5/1024 = 0.00488 volts ( 4.88 mV). We know that the sensor outputs 10 mV per °C, so how many steps in the ADC output would indicate 10 mV?

We see that the 10 mV divided by 4.88 mV gives us 2.04918 steps, but the ADC steps are integers so we are going to have to do some math here. Two steps of the ADC ( 2 4.88 mV) equal 9. 77 mV, so two steps are pretty close to a single °C change in temperature ( 10 mV). Notice that the error between 10 and 9. 77 is just under 1%, and since the error for the sensor itself is 2% we might be safe ignoring it and saying that two steps in the ADC output is equal to exactly 1°C.

Doing that would allow us to use integers rather than floating point data (a data type that has integer and fractional parts divided by a decimal point), which uses a