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# OCR Physics AS Planning Exercise watch

1. Why do you need a resistor at all, surely you could just put a voltmeter around the photocell. That would give the ouput of the photocell.
2. can you use a LDR to recieve from the infrar-red source or would you use a phototransistor?
3. (Original post by punit)
can you use a LDR to recieve from the infrar-red source or would you use a phototransistor?
the answer is no because LDRs are not sensitive to waves with wavelength greater than 750nm and as you may be aware, the average wavelength of IR is 850nm... so LRD will be useless for the purpose...
i would advice you to use either phototransistor or photocell... someone disscussed about that here...
4. (Original post by Meat Loaf Rocks)
Why do you need a resistor at all, surely you could just put a voltmeter around the photocell. That would give the ouput of the photocell.
I think the purpose of the resistor is to act as a potential devider... but 98% sure that the circuit diagram should include resistors...
5. but why do you need to measure the voltage at all? surely you can just measure the current with an ammeter in series with the photocell?when it says output, do you think it means voltage or current?
have any of you considered any other infra red radiation in the room? or just ignored it?ive thought about doing it in a dark lab...but im not sure
6. I am going to be setting up the equitment like this.

tho i'm not sure what to use as an IR source, or what else to write about i've only doen 280ish words but i thik i've coverd it all?
but why do you need to measure the voltage at all? surely you can just measure the current with an ammeter in series with the photocell?when it says output, do you think it means voltage or current?
have any of you considered any other infra red radiation in the room? or just ignored it?ive thought about doing it in a dark lab...but im not sure
It just asks for an output. Curreent is an output as is voltage. E=VIT hence the energy detected is proportional to Current or voltage, so it makes no difference.
8. http://www.maplin.co.uk/free_uk_deli...itter_2262.htm

there you go buddy
9. (Original post by Spikas)
tho i'm not sure what to use as an IR source, or what else to write about i've only doen 280ish words but i thik i've coverd it all?
Use IR emitting diode as your source and to make up rest of the words, expand on your procedures section... have you done the risk assessment? and you may also want to expand on the background info... hope this helped
10. if using a photoresistor that has changing resistance when different levels of infrared hit it, wouldn't just an ohmmeter plugged into the photocell to measure the resistance do?

know voltmeters are in parallel, and ammeter in series. how do ohmmeters fit in?
11. i think if you use an ohmmeter you have to take the component out of the circuit...? i'm not sure though.

it'd probably be easier to just measure the output voltage...
12. Would you use DC supply or AC supply?, i thinking DC, am i right
13. (Original post by punit)
Would you use DC supply or AC supply?, i thinking DC, am i right
AC I guess... coz thats whts used in the "real world"... but it doesnt really matter...

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