How Does Distance Affect Photocell Output?

[Hootenanny]

bigbadcityboy,

Photovoltaic cells produce current not a potential difference. May I suggest you read around the subject a little, may I also suggest that you start here; http://en.wikipedia.org/wiki/Photovoltaic_cell.

And you friends will not be measuring voltage unless they are using a current to voltage converter.

~H

 

[super_swifty]

okay, i have read through the thread and firstly i would like to thank hoot and berke for their patience and help. i think it was really unfare (and illegal) of einstein to ask you to write his report for him.

where could i find a type of Light Detector suitable for detecting a infrared source from approx 2 metres away

 

[berkeman]

where could i find a type of Light Detector suitable for detecting a infrared source from approx 2 metres away

What's the application? What's the light source, and its intensity and directivity? How big can the detector be? Can it use light concentration? Are there other interfering IR sources in the same environment? Will you be using modulation/demodulation to reject those other IR sources?

The simplest thing would be to just use an IR remote control transmitter and receiver circuit, along with the IR window for the receiver assembly. You can probably even get them in kit form...

 

[berkeman]

bigbadcityboy,

Photovoltaic cells produce current not a potential difference. May I suggest you read around the subject a little, may I also suggest that you start here; http://en.wikipedia.org/wiki/Photovoltaic_cell.

And you friends will not be measuring voltage unless they are using a current to voltage converter.

~H

And I'll add that I seem to remember way back somewhere in this thread that the goal was to measure the output of the detector at different distances from the source, in order to figure out how that varies with distance. (Was that this thread?) You can go ahead and measure the output voltage of the photocell, but it's not going to vary much or vary smoothly with distance. When you do a good job of measuring the photocurrent (however you end up doing it), that is when you will get a nice smoothly varying function with respect to the distance from the light source.

 

[berkeman]

Big Up The Physics Masseev Wicked

You talk just like my son! You aren't berkeboy in disguise just messing with me, are you?

 

[Zalaras]

Would a remote controller for a TV be a suitable infrared source for this experiment? I've edited this because, stupidly, I did not read enough above.

 

[berkeman]

Would a remote controller for a TV be a suitable infrared source for this experiment? Is there going to be a significant change in the photocell output over the distance of about 1.5 metres?

The IR LED from the transmitter would be a valid source, but you will probably want to force it on instead of leaving it transmitting in code. However, keep in mind that TV remote control receivers use the "current-to-voltage" converter circuitry that hoot and I have alluded to (multiple times), and they have significant gain and AGC and decode circuitry to make it possible to operate over the several meter distances. You're not going to be able to just hook up a DVM in current mode, and see a photocurrent at an IR photodiode from an IR LED a couple meters away.

 

[berkeman]

i always thought u couldn't put an ammeter across a battery, am i wrong?

An ammeter is a short circuit (in reality a low explicit resistance), so hooking it across a voltage source is looking for trouble. You will likely blow the fuse in your ammeter (or DVM's current measuring circuit). And no, you can't borrow my DVM for the experiment.

 

[Zalaras]

Thanks for the info.

I'm in the process of writing up my Scientific Knowledge section and I was wondering if anyone knew of some good sites for information about infrared radiation and photocells in relation to the stated experiment. I've tried some sites but I can't seem to find the type of info I'm looking for.

I have made some predictions based solely on guesses so I really need to check out some more information so I can refine these predictions.

Thanks in advance.

 

[berkeman]

I googled IR photocell tutorial, and got lots of good hits. Have you already read through these?

http://www.google.com/search?hl=en&q=IR+photocell+tutorial

 

[Zalaras]

About the reply earlier. Say I was to set up a circuit with a photocell and ammeter in a dark room and shot IR radiation from a remote control at the photocell. Would this give a good set of results? How would I be able to see the readings in the dark ?

I have not seen those sites but I will make sure to have a browse through. Thank You .

 

[berkeman]

i couldn't find any data sheets for solar cells on the digikey thing. I am assumin' itl output something in the mA range

Well, I just now googled solar cell datasheet, and got a bzillion hits. I went down into one, and it had the J = xxxx mA/cm^2 type number that you are looking for. Check the hits out to see if you find what you need:

http://www.google.com/search?hl=en&q=solar+cell+datasheet

 

[Rufio]

Ive already completed this exercise and handed it in and I'd just thought I'd tell all those still doing this that this is a "planning exercise". Therefore, they are testing your ability to plan an experiment, not necessarily your ability to understand theory. A little theory may be required to explain why you are doing something, but a lot of information on this thread would gain you no extra marks. Remember, its only 2 marks per point (as Einstein wrote earlier) and this is only worth about 5% of your total grade! A thesis is not required or expected!

 

[Cheery Blossom]

Thanks for the consolation Rufio-Chan

 

[Rufio]

No problem! All you've got to remember is to not leave out the simple physics. They are the bits that'll give you marks!
And a quick tip: An empty table of results is a very handy way of showing what you are going to measure, in what scale, and does not count in the word count! (I don't think there is a specified word count but the recommendation from the exam board is between 500 and 1000 words)

 

[PenguinBoy]

if i put a resistor into stop the components from overloading, would that stop the ammeter readings from being inversely proportional to the square of the distance?

 

[Hootenanny]

if i put a resistor into stop the components from overloading, would that stop the ammeter readings from being inversely proportional to the square of the distance?

No. You wouldn't change the relationship, you would just 'shift' it, similar to translating a curve.

~H

 

[duck86]

Hahah poor little einstein, like many others i also have the misfortune of doing this planning exercise, but then again i also have the misfortune of re-doing my a-levels when I am 20 (Buggery). Its quite a tasking one, but with some thought its easily done. Some of the points raised on this discussion are far beyond what is needed for such a simplistic experiment. I can only suggest to think of the ideas you have been taught this year and not look for answers which you can't comprehend. It gave me a huge headache for a few days, but I am now smiling again.

 

[testament]

hey, i was wondering if u think it would be sufficient to make a potential divider with a pull up resistor and measure the output voltage across the photo detector and if I should use it to work out the current if that would be more accurate, thanks

 

[berkeman]

hey, i was wondering if u think it would be sufficient to make a potential divider with a pull up resistor and measure the output voltage across the photo detector and if I should use it to work out the current if that would be more accurate, thanks

See posts #28 and #62.

 

[testament]

oh, sorry, luckily I'm not being tested on my observational skills, having now read the whole thread you've pretty much done the work for us, reading wot everyone's posted on here i don't think there's a single brain cell between us!

 

[evrox]

ok so I am going to have a photocell in series with a resistor and I am going to mesure the pd( voltage) of the resistor. ok this is really stuid question but I am confused. when the photocell receives more light its resistance drops so does that mean that the pd across the resistor will increase?

 

[evrox]

so I am therefore going to have a negative gradient pd vs distance graph? please help! I am having a temporary momeny of insanity

 

[berkeman]

A photocell or photodiode is not a "light dependent resistor". An LDR is constructed differently, and is probably based on more of a CMOS structure. A photocell or photodiode is a bipolar PN junction, where incident photons liberate electrons that form the photocurrent. The fundamental "thing" that you get from a photocell is current, not resistance or voltage. The fundamental thing that will drop off as the receiving photocell is moved farther from the light source is the photocurrent.

There are different ways to measure the photocurrent, and you should take a look at a typical DVM to figure out whether it will give you the best range of readings when it is set to a current measuring scale and connected directly to the photocell, or if it will work better if you connect a resistor across the anode and cathode of the photocell and measure the voltage generated by the photocurrent passing through the resistor. I honestly don't know which would be better with a standard DVM. You need to do the math to figure that out.

 

[evrox]

ive spoken to my physics teacher and this is definitely the way to do it. I am using a cds photocell.

 

[super_swifty]

for i LDR i would use a multimeter and measure the resistance. (would i need a current for this?)

 

[Hootenanny]

for i LDR i would use a multimeter and measure the resistance. (would i need a current for this?)

As far as I know you would be required to connect your LDR to a power source before measuring the resistance. I could not say what would happen to the relationship, whether or not it would be altered.

~H

 

[Iamphoebe]

I've been reading through this thread over the last few weeks, and it has provided me with really valuabe information - just about the only info I could find. I can see that, although i am confused myself, it is frustrating for the people offering advice. thank you all so much for your help and patience. (and good luck everyone)

 

[Hootenanny]

I've been reading through this thread over the last few weeks, and it has provided me with really valuabe information - just about the only info I could find. I can see that, although i am confused myself, it is frustrating for the people offering advice. thank you all so much for your help and patience. (and good luck everyone)

Thank-you phoebe, it is nice to be appreciated . It was a pleasure. I also echo phoebe's best wishes for anyone involved in this planning examination.

~H

 

[lining515]

i hve read through all the threads...
it seems quite useful...but it really confused me ~
coz we hve been doing some preliminary work in class, and all we did is to connect a radient heater to a power supply...and it emitts out infrared and we connected da photocell with a voltage sensor and a data logger and measured with different distance and figured out da inverse square law relationship..i just don't understand why you can't not connect a voltagmeter directly across da photocell..because our physics teacher suggested us to do it..so ?
and all we need now is da various distance?and homework to produce da point source of infrared...im thinking of conver da radient hearter with sum sort of card and make a hole underneath it so it emitting out infrared frm tat pinhole~~