Science Buddies: "Ask an Expert"

My son encountered an issue with this experiment. The voltage across the 10 ohm resister is 3.04. This value remains 3.04 regardless of the distance between the light-to-voltage converter and the LED. In fact, he receives the same value for the presence of any light. Covering the light-to-voltage converter completely or turning off room lights will reduce the voltage measurement to 0 volts. I purchased two TSL257-LF converters (Mouser Electronics) and experience the same results for both. Is this component too sensitive to ambient light? Any suggestions are appreciated.
Thanks

Hi luvstem,

I believe that should be a 10 kOhm resistor (I see that the guide has a typo and will get that fixed), although in theory, that shouldn't change your result.

It does sound like the circuit is too sensitive to ambient light. If you want to send an image of the circuit, I can also take a look.

Best,
Charles

Charles,
thanks for the quick response. Yes, meant to write that the circuit includes a 10k ohm resistor. Reviewing the forum over several years, it appears that the light to voltage converter part number may have changed. Below is a link to the circuit that my son used for the experiment. I'd appreciate your feedback.

https://photos.app.goo.gl/yFNNUMkwsxoSpGU7A

Hi Luvstem,

I'll jump in to add that there are now many free apps available for your phone that can measure light levels - this could be a good substitute if you continue to have trouble with the light sensor circuit. We have a tutorial for one such app, Google Science Journal, here:

https://www.sciencebuddies.org/science-journal-app

but you can find others by searching for "light meter" or "lux meter" in your phone's app store.

I glanced at your photos and your circuit does look correct. If the sensor is only outputting the extremes of 0V or 3V then that does seem to indicate that it's saturating. If you only partially cover it with your finger, can you get it to read anywhere between 0 and 3V?

edit: here's the direct link to our page about Science Journal and the light sensor: https://www.sciencebuddies.org/blog/goo ... ght-sensor

Thanks for the links to the phone app and the sensor page. We will review the information and give them a try.

We tried partial covering with no success. Simply leaving a dimmed light in the room was sufficient to saturate the converter. We have a second light to voltage converter and we'll try it.

Regarding the sensor app, are you proposing that we measure the lumens, convert the measured value to power (watts) and use that as the Optical output power of the LED and Incandescent lamp?

I found two conversion formulas

Power(W) = ΦV(lm) / η(lm/W) ,where lm equals luminous flux in lumens and η is the luminous effeciency (80 - 100 range for LEDs)

So

watts = lumens / (lumens per watt)

Hi - sorry I didn't get into details about the units. The Google Science Journal app measures light intensity in lux, which is equal to one lumen per square meter: https://en.wikipedia.org/wiki/Lux. I'm not sure if it would be easy to convert directly from lux to watts in this case (I would need to look more into the formulas and figure out if it requires knowing the area of the phone's light sensor). However, I don't think it's necessary. Notice in the original procedure how equation 2 has an "N_linear factor" in it - there's some linear factor that would convert the measured resistance to watts, and you don't actually know what that factor is. At the end of the day, if you want to calculate the ratio of output power between different types of lights, it doesn't matter because the N's all cancel out. It would be the same case here - you can assume power is proportional to lux (at a fixed distance) and there's some conversion factor between the two. You don't actually need to know what that conversion factor is, as mentioned in step 4 of the Testing and Data collection section of the procedure: "This project assumes that optical output power is the power collected by the detector. Using this assumption, N is the same for both the LED and the incandescent light sources, and the output power can be written in units of N in the data table (for example, write 0.75 N in the table). N cannot be determined easily, as it depends on the light emission vs. angle for each source."

Thanks for the additional information. Looks like we have the opportunity to try a couple of different things. My son has a lux meter that he used for a prior LED experiment, so we'll use that as well as the Google smartphone app. He has an idea to limit the ambient light and use the procedure as written, so we'll give that a go also. we're having a lot fun and he is learning a lot due to the challenges.
Thanks for the responses and guidance.

That's great to hear that your son is having fun and learning a lot despite running into some obstacles. That's a fantastic real-world lesson that some students tend to get frustrated by - science doesn't always work out nicely on the first try! The troubleshooting process itself is a valuable learning experience. Please write back if you have more questions!