ATMS 748 Homework and Course Deliverables (return to main page)
[How to write lab report]


Assignment 3
Title: Quantitative Generation and Measurement of Light with Arduinos, LEDs, and Photoresistors.

The goals of this assignment are:

a. Explore the use of photoresistors, LEDs and photodiodes as light detectors for applications.
b. Introduce a very common, and extremely useful device for measurements, the operational amplifier.
c. Show how you can get quantitative measurements from the Arduino microcontroller.
d. Reinforce the difference between ideal voltmeters and real voltmeters, that meter input resistance matters.

Measurements and Analysis
1. You have already done circuit 6, pg 40 in the book to learn about the photoresistor and how to measure its output.
We are going to add to that circuit.
Add a circuit to drive an LED to see if the photoresistor resistance can accurately follow the LED output for low and high frequencies.
Point the LED output directly into the photoresistor input. You can use variable delay and the 'Blink' sketch to drive the LED.
If the LED is driven by a square wave, the photoresistance should show a crisp square wave too. Use the plot monitor on Arduino to view the photoresistor output,
and save some data with CoolTerm (including time) so that you can graph the photoresistor output from the LED drive as a function of time.
Use Excel to obtain the time constant of the photoresistor as a sensor of light.
Here's an example sketch that may be helpful in measuring response time. You may find ways of speeding it up to get more time resolution on the LED response.
[Does photoresistor nonlinearity play a role in its frequency response?]

Description of the circuit for the photoresistor test. Click on the image for a larger version.



2. Build a simple circuit with a resistor and LED as a photodetector so that you can measure the photocurrent from the LED such that 0.1 microamps produces a 1 volt signal.
What resistance value are you going to need to 'program' the voltage output this way?
Construct transimpedance amplifier circuit with an op amp to convert the photocurrent from an LED as a light detector to a voltage so that a photocurrent of 0.1 microamps produces a 1 volt signal.
A. Write a sketch (or modify the sketch from part 1) for the Arduino to output relative time, and voltage from these circuits with as fast an update time at you can make.
You may need to increase the serial communication rate with the computer and write your code so that the Arduino makes efficient use of computing time .
Use a light source and rapidly shine it over the detectors.
Write your data to a file using either hyperterminal for the old computers, or another serial port capture program for other systems.
Graph your time series. Compare the time response and noise of these circuits.

Board for the Op Amp lab. Click on image for larger version.

Photograph of the Arduino set up for this lab. Click on image for larger version.

3. We will do this exercise in class as a group.
Use the fast ultrabright LED light source to drive both circuits at the same time with a square wave using the function generator.
Record the waveforms for both circuits simultaneously for frequencies of 10 Hz, 100 Hz, 1 kHz, 2 kHz, 10 kHz, and 100 kHz.
Record the waveforms using the oscilloscope connected to the computer.
It would be good to make sure the room lights are off if interference happens.
Discuss the effect of the circuit time constant on the ability for the detecting circuit to follow slow and fast response circuits.
Estimate the LED capacitance.
Discuss the difference in noise for each circuit.

4. Extra credit: One idea is to use the LEDs as detectors to estimate/measure cloud optical thickness.
[idea for additional project: measure the downwelling solar irradiance with both blue and red LEDs.
On a clear day the blue channel should receive more radiation than red compared with a cloudy day.]

OSRAM BPW34 photodiode data sheet.
Discussion of operational amplifiers.
Use of light emitting diodes as wavelength specific radiation detectors,
as they are very useful for use in simple sun photometers, among other applications.
Description of the Arduino and some sensors we'll use.
Very useful voltage divider circuit to use for measuring sensors that depend on resistance.

Click on image for larger view.





Assignment 2

Purpose: Gain an overview of atmospheric measurements. This assignment is given in webCampus.



Assignment 1

Become familiar with the Arduino microcontroller, breadboarding circuits, and obtaining your own measurements with an analog to digital convertor.
Establish a common set of expectations for lab report writing.

Do all 12 projects in the Arduino guide book. There may be errors in the projects, as we'll find out as we go along..
I strongly recommend you install the Arduino software on your own laptop, and use it in class, if you have one.
The code for the projects is here: expand the file and put the folder in your Arduino examples folder.

Write a report about 3 of the experiments, temperature measurement, light measurement, and shift register control of 8 LEDs.
Your report needs to include the following (and see below for general reporting requirements):
What is the Arduino?
What is the principle of operation of the temperature sensor? How was its signal obtained?
What is the principle of operation of the light sensor? How was its signal obtained?
What is a shift register? How and why are they used?

Final open ended question: What would you like to do with the Arduino and appropriate hardware (you specify), if given the time and resources for development?


Description of the Arduino and some sensors we'll use.

Description of the analog to digital conversion for the TMP36 sensor. Click on image for a larger version.
These notes are for attempting to increase the
resolution of the measurements.


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