Help needed on photocells please.

[Cheery Blossom]

lewmang, u'll have to ask your physics teacher about that...

 

[lewmang]

ok, also what kind scale of voltmeter would be needed and how would i know what scale to use? I will set the powerpack to let's say 6V on each circuit.

 

[lewmang]

would a voltmeter that reads to 5V be ok?

 

[lewmang]

please answer it has to be in for tomorrow

 

[berkeman]

So what photodiode/photocell/LDR did you pick for your project? What does the datasheet say about the output characteristics of the device? That will guide what instrument you use for the experiment, and what kind of accuracy you can expect, and what distance ranges you can expect to cover. Do the work.

 

[lewmang]

there is no datasheet, the physics teacher just said we can use an ordinary photocell, maybe i should just say i will choose an according voltmeter after tryin the experiment with a 0-20V voltmeter

 

[berkeman]

Well, you need to do this. Go to www.Digikey.com and type photocell into the search box. You will get 32 hits, sorted by output type. You can apply some filters to the results if you like, and in the end, download and *read* a couple of their datasheets. You might even do something radical and note the vendor name, and go to their website and look for Application Notes or other information that will help you write up a cohesive and knowledgeable report. That's how we do it in the world after school, so it's good for you to start getting some practice on it. Good luck.

 

[lewmang]

i don't understand what a 20K Ohm at 10 lux photocell, all i know is i think it creates a voltage when light is shone on it

 

[lewmang]

does the circuit with the photocell need a power supply or does the photocell produce a flow of electricity?

 

[Hootenanny]

i don't understand what a 20K Ohm at 10 lux photocell

This means that when the photodiode is exposed to light of intensity 10 lux it has a resitance of 20K ohms. This is alos its maximum resistance.

does the circuit with the photocell need a power supply or does the photocell produce a flow of electricity?

Depends what circuit you are using, if you are connecting your photodiode directly to an ammeter, then no power supply is required, however, if you are using a current to voltage converted (Op amp), then you will need a supply.

~H

 

[lewmang]

can i connect a voltmeter directly to the photocell?

 

[bigbadcityboy]

im using a photovoltaic cell, would it be ok to measure the voltage with this with a circuit with just the cell and a voltmeter?

 

[berkeman]

im using a photovoltaic cell, would it be ok to measure the voltage with this with a circuit with just the cell and a voltmeter?

No. You need to measure the photocurrent.

 

[bigbadcityboy]

but a solar cells voltage output stays the same but it outputs a curent?

 

[bigbadcityboy]

surely the 'voltaic' part of photovoltaic cell means it outputs a voltage. otherwise why would it be called that?

 

[berkeman]

surely the 'voltaic' part of photovoltaic cell means it outputs a voltage. otherwise why would it be called that?

Please read up a bit on how photocells and photodiodes work.

And stop calling me Shirley.

 

[berkeman]

whats an explicit resistor?

It's one that's R-rated. Like the movies.:rofl:

Sorry, I'm just getting a little punchy with this long repetitive thread about fairly simple stuff. For the purposes of this thread, just ignore the word explicit.

 

[bigbadcityboy]

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

 

[bigbadcityboy]

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

 

[bigbadcityboy]

help! all my friends say theyr measuring voltage! they say a photovoltaic cell is like a battery so makes voltage not current

 

[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 :tongue2: ?

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 :tongue:

 

[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)