Nuts and Volts - November 2014

62 November 2014

lot more of the computer’s resources to do calculations that do integers. Using the integer simplification would probably be good enough for most applications and would save us some serious computing. However, since we aren’t doing a lot of extra work with this program, we will go ahead and write the code using floating point data types. (To rephrase: In a more constrained program that does a lot of work and takes up a lot of space, we probably wouldn’t use floating point, but accept the error and say that two ADC steps equals 1°C.)

Or, saying this another way, we know that the temperature sensor will output voltages between 100 mV and 1,750 mV to indicate - 40°C to +125°C. The ADC outputs values from 0 to 1023 in steps representing 4.88 mV per step. This tells us that the lowest valid ADC reading will be for 100 mV giving us: 100 mV/4.88 mV/step = 20. 49 steps. Of course, these steps are integers so we will say that the lowest valid step value is 21, which equals 21 steps 4.88 mV/step = 102.48 mV.

We know the temperature sensor outputs 100 mV for - 40°C, so 102.48 mV is slightly above that. We know that the sensor outputs 10 mV per °C, so we can calculate 2. 48 mV / 10 mV/ °C = .248°C, giving us a temperature of - 40°C + .248 °C = - 39. 75°C. Of course, our real accuracy isn’t quite that good, but for our purposes here, we’ll say that our systems lowest accurate temperature is - 39. 75°C.

We can repeat this logic to calculate the highest accurate temperature reading: 1750 mV / 4.88 mV/step = 358.61 steps (359 steps since it is an integer). However, when we calculate 359 steps 4.88 mV/step = 1751.92 mV, we see that the 359 takes us above the highest accurate temperature output (that is, 125°C at 1750 mV), so let’s drop the maximum step to 358 and calculate 358 steps 4.88 mV/step = 1747.04 mV, which is in range.

We now see that the range of accurate readings from our ADC will be 21 to 358 out of a possible range of 0 to 1023. For our purposes, we should consider any ADC readings below 21 or above 358 as invtalid, and ignore them when we do our software conversions of the ADC readings to voltage and then to °C.

Since this discussion is fairly complex, we will have a lab to use all of this in a test program to convert fake ADC readings to voltage and °C.

Lab 1: Sensing Relative Light Levels

This lab implements the light measurement discussed previously. This circuit works like the one back in Figure 3, except we will use our hand instead of clouds to vary the light falling on the sensor. We will then use the Arduino ADC to measure the changing voltage across the resistor.