PR Experiment 2: Light Intensity vs Resistance Output - reynold5/ECE387_Midterm GitHub Wiki

Purpose: The purpose of this experiment is to correlate how light intensity (measured in lumens) directly effects the resistance output of a photoresistor. Prior to beginning this experiment, it was determined that the 200 lumen flashlight would be held at a distance of 18.5 inches away from the photoresistor. This distance was determined as a constant because it generated an approximate 200 lumen reading on the light meter, without any shielding. This constant dictates that the highest intensity we will be able to measure in this experiment will be 200 lumens, the same value as the flashlight output.

Materials Used:

  • Arduino Board/Arduino Software
  • Breadboard
  • Photoresistor
  • 10k Ohm Resistor
  • 3 Wires
  • 200 Lumen Flashlight
  • Dark Colored Foam Board
  • X-Acto Knife
  • Ruler
  • Light Meter (I used the "Light Meter" app on iOS for iPhone)
  • Very Dark Room
  • PR Simple Circuit Source Code

Procedure:

  1. Arrange the photoresistor and the 10k ohm resistor as illustrated in the "PR Simple Circuit Source Code" document located in the "Code" tab above.
  2. Copy the source code provided into the Arduino software, compile the program, and upload it to the microprocessor.
  3. Ensure your Arduino microprocessor and attached circuitry are relocated to a very dark room. Ensure the measurement panel on the Arduino software is displayed.
  4. Using your ruler, measure the diameter of the face of your flashlight (my flashlight face was 3.60cm). In 0.25cm increments, make square cutouts in your foam board that increase by 0.25cm increments until you've made a cutout the same diameter of your flashlight. Below is a figure of the desired outcome.
  5. In the dark room, arrange the foam board so that the desired cutout is 18.5 inches directly above the photoresistor. Place the light meter as close to the photoresistor as possible without impeding the light source from the photoresistor.
  6. Pressing the flashlight firmly against the board, shine the flashlight through each cutout and record both the light intensity (via the light meter) and the resistance value displayed on the measurement display within the Arduino software.

Results: The following figure correlates the relationship of light intensity (in lumens) and resistance output of the photoresistor (in Ohms).

Discussion & Limitations: As you can see from the graph, as the intensity of light increases, the resistance output of the photoresistor decreases which is what we expect to see in terms of normal photoresistor operation. In regards to the graph, areas with a steeper slope indicate a more drastic response to the change in light intensity. The most drastic response takes place in the range of 0-50 lumens, a range of very low light levels and the change in resistance is less drastic in areas that are already well lit (normal daylight is considered to be around the 100 lumen mark).

One major limitation to this experiment was the lack of a professional light meter. Since a light meter application was used, detecting very precise and specified light levels was very difficult, only certain increments could be obtained due to the sensitivity of the meter. The measured intervals of light intensity are not equal because of this limitation. Much like the sensitivity of the photoresistor, the light meter was less sensitive to light intensity above the 100 lumen mark, this is evident in the spacing of the measurements taken.