Worksheet Exercises

Task

We are to create a program to read from a temperature sensor and a light sensing circuit.

Hardware Setup

Fritzing schematic: apsc160_analog.fzz

Component	Count
Full Size Breadboard	2
Photoresistor (approx. 10 kOhm - 30 kOhm)	1
Temperature Sensor (TMP36 or LM35)	1
Slide DIP Switch (2 or 4 positions)	1
10 kOhm Resistor	2
Shift Register (74HC595, 14-DIP)	2
4 digit 7-Segment LED Clock Display (Common Anode)	2
220 Ohm Resistor	8
Spindle of Wire (22 Gauge Solid)	1

Analog Value Conversion

To obtain meaningful results from our analog measurements, we need to convert the raw analog values that come out of the Arduino's analog-to-digital converter.

Analog to Voltage

On the Arduino, the analogRead(...) function returns a value in the range of [0, 1023], where 0 corresponds to 0 V and 1023 corresponds to the reference voltage (5 V by default). Thus, to convert the raw value to voltage, we must apply $$V = \frac{a}{1023}V_\mathrm{ref},$$ where \(a\) is the raw analog value, and \(V_\mathrm{ref}\) is the reference voltage of 5 V.

Temperature

The TMP36 temperature sensor has voltage-temperature characteristics described in the specifications sheet as Thus, we can compute a temperature from the output voltage using the equation $$T = 100 (V - 0.5),$$ where voltage is measured in volts, and temperature in degrees Celsius. To convert Celsius to Kelvin, we add 273.15.

Illumination

The light sensing circuit is created using a voltage divider with a 10 kOhm resistor and a photocell (also referred to as a photoresistor). The datasheet for the photocell specifies a linear relationship between the log of the resistance and the log of illumination: $$\log(L) = M \log(R) + b,$$ where \(L\) is luminance measured in lux, and \(R\) is the resistance of the photocell. The output voltage of the voltage divider is given by $$V = \frac{R_D}{R+R_D}V_\mathrm{ref},$$ where \(R_D\) is the 10 kOhm resistance, and \(V_\mathrm{ref}\) is the reference voltage of 5 V. Putting these equations together, we have $$L = \exp\left[M\log\left(R_D[V_\mathrm{ref}-V]/V\right) + b\vphantom{\sum}\right]$$ By measuring luminance in a variety of lightings using a free phone application, as well as the corresponding output voltages of the voltage divider, we experimentally determined the parameters for our particular photocell as \(M = -1.4\), \(b = 15.76\). These values may need to be calibrated for your photocell (e.g. see this tutorial).

Software Implementation

For this program, we provide a library to handle the displayWrite(...) functionality, since this is not part of the Arduino API. This library assumes the exact hardware configuration as given in the Hardware Setup above.

• Header: DisplayWrite.h
• Implementation: DisplayWrite.c

To add these files to your program in the Arduino IDE, download them somewhere known on your computer, and go to "Sketch > Add File..." and select both files. In Visual Studio with Visual Micro, right-click on the project and select "Add > Existing Item..." to add the both the header and source files.

analog.ino