Sliding light... light meter problem?

[deleted-189284]

I have everything for my project but i cant find a light meter and i cant order it because my sci fair is in 3 days! i just want to know what can i substitute for light meter and if not anything then is there any other way to do this project???? i am grade 9 is this project good?

[deleted-249560]

Without a light meter this project could be a tough one. If you're familiar with cameras and understand the relationship between shutter speed, aperature and light levels you might be able to come up with a methodology to do this project with a camera that has manual settings.

For example, if I have a camera set so that the meter shows a correct exposure at f8 and 1/30 of a second and then change the camera to 1/60 of a second, I'll need exactly twice as much light to get the right exposure. In reverse, if you light a scene with your lightbulb and find camera settings that could take a picture in that light, you could then change the bulb brightness with your slider and find new camera settings. Compare the two camera settings and you'll get a *relative* amount of light difference. It's not the same as a lux meter but it might do the job if you can explain why that works.

Alternatively, you have a multimeter (I saw your other question) and if you can find a CdS photocell at a local Radio Shack or other electronics store, you can use that cell as a meter. It changes resistance more or less linearly with light. There's no calibration to compare to any real numbers though, but again it will allow you to make a relative measurement - 'this one is twice as bright as that' sort of thing.

You'll have to get creative and make up your lux meter alternative yourself but hopefully one of these two appraoches will give you a starting point.

Howard

[deleted-189284]

thank you so much! i liked the second suggestion you gave about the CdS photo cell..do i have to attach it to the multimeter and then place it near the light?
and i also want to know that if i can download a lux meter app on my ipad and then click pictures of the light.. will that help?

[deleted-249560]

A photocell will act like a resistor that you can read with your multimeter using the resistance or Ohms measurement. In the dark it will read higher than it does in the light. One way you can do this is: in a dark room make your tests using the pencil. Place the photocell on something that won't move and place it near the bulb. Remember that the distance and positioning are critical so it's important that the room light and arrrangement of your test setup doesn't change. When the bulb is lit, look at the meter and note the resistance. Change the bulb's brightness by sliding on the graphite and look at the meter again. If the bulb is twice as bright the resistance will be more or less half the resistance. I say more or less, because a CdS cell is approximately linear but not quite. Still, in a pinch it's better than nothing for your application.

There are several lux meter apps you can download from the Apple store. I have no experience with any of them so you're on your own there - but for the purposes of this experiment - and given your short time frame - that might be a good alternative. Please write back with your experiences if you decide to do this with an app. Other students might benefit from your feedback.

Howard

[deleted-189284]

i am deciding to go with the app...
but for the cds photo cell do i have to attach it to the multimeter, how to do that?
and also is the cds photo cell easily available??

[deleted-249560]

You set your multimeter to measure resistance or ohms. Plug in the red and black leads to the meter as marked - the black lead will go to common as usual and the red lead probably goes to the same connection you use for volts. The CdS cell used to be available at Radio Shack but you can certainly get them at any electronic distributor. Just connect the cell leads to the meter leads. It doesn't matter which one you put on red and which you put on black. Select a range that displays numbers or put the meter in autoranging. That's it.

The cells look like this: http://comingsoon.radioshack.com/cds-ph ... VpiFkYmkc4 or this: http://www.jameco.com/webapp/wcs/stores ... _120299_-1

Howard

[deleted-189284]

Thank you so very much for this help.
but how do i convert this measurement in resistance to illuminance? and also is this project okay for a grade 9 student?

[deleted-249560]

Converting those readings to a standard measurement is the problem. You can't just do that without acquiring some calibration data by measuring the same light setup with both your CdS cell and a light meter. And of course, if you could do that you'd have a light meter.

So you can't actually get illumination in anything like lux but you can say that the light at one setting is twice as bright or 1/3 less bright than at another setting. In theory the lux meter app you pick will have done some calibration and will present data you can trust. It may not be terribly accurate but again it could be amazingly good. Without comparing the readings to a known working lux meter you can't know.

Howard

[deleted-189284]

SO i have decided to use the app. .! my last question is that do i have to follow the engineering process or can i follow the scientific method (i prefer scientific method)?

[deleted-249560]

You're not designing anything - you're making controlled measurements after manipulating one variable. This is a scientific method project if you're measuring the light - no question.

Good luck and have fun with your project! Do let us know how the light apps work.

Howard

[deleted-189284]

Okay thank you very much!

[deleted-189284]

Hello,
Can anyone tell me what is the constants and variables for "Sliding light: how to make a dimmer switch" project?

[deleted-249560]

We love helping out, but this one you'll have to work on a bit yourself.

Start by looking to the resource here: https://www.sciencebuddies.org/science- ... bles.shtml

Short answer- the constants are those things about the experiment that don't change. The independent variable is the one thing you change and the result is the dependent variable.

So you've read through the procedure and understand how the experiment is done. What one thing did *YOU* change? What changed as a result? What about the experiment (setup, materials, environment) stayed exactly the same as you did the experiment?

Howard

[deleted-189284]

Thank you so very much!