LED Lighting and PmodALS

[fishes_alot]

I was "playing" with the PmodALS Light Sensor in my home office.  The program was set to output the light sensor reading once each half second and these are the readings I saw: "27, 27, 23, 28, 10, 30, 4, 32, 5, 31, 10" .  Needless to say, I was disappointed with the fluctuations of the light sensor output readings.

 

Then I remembered that I had recently changed my office lighting to efficient "dimmable" LED light replacements.  I began to suspect that the sensor is fast enough to take a sample when the LED lights are off. So I turned off the office lights and illuminated the sensor with a flash light. The readings now were "44, 40, 43, 57, 57, 56, 53, 50, 49, 49, 46, 41, 32".  I am inclined to attribute the fluctuations in the sensor readings to my unsteady hand holding the flashlight. This is not the highest quality experiment but it did give results consistent with my theory. Has anyone else experienced this kind of PmodALS behavior?

[JColvin]

Hi fishes_alot,

 

I have seen very similar behavior with the PmodALS.

 

As you suspect, the sensor, or more specifically the ADC that takes the voltage from the light sensor and gives you the 8-bit value, is fast enough to catch when the light is on or off. According to the datasheet of the ADC, it can collect data from 50k samples per second up to 200k samples per second (50 kHz to 200 kHz), making it plenty fast enough to notice if a light is flickering. The light sensor itself is also sensitive to angle changes; I can shine a bright LED on the Pmod and get a consistent reading, but if adjust the Pmod very slightly (about 3 mm) so that the spot of light isn't shining as directly on the sensor, I can see a change of 10 or more. You might see more or less of a change depending on how close or far away (respectively) your light source is from the light sensor component.

 

However, if you have the PmodALS laying on a flat surface or as long as your hand isn't visibly shaking and it is receiving light from a steady light source (such as a ceiling light), it should read out very consistent values. Naturally, I can't see how you're holding the Pmod or the light, but it could be that the fluctuation you're seeing is because of those small angle changes, especially if you're using a distinct spot of light (like a flashlight) instead of a "wide area" light.

 

Let me know if this helps.

 

Thanks,

JColvin