How Does Distance Affect Photocell Output?

[lewmang]

Voltmeter?

Hi, I am doing the planning exercise aswell.

Would a voltmeter be ok to measure the output of the photocell because voltage is proportional to resistance ( which varies with amount of infrared radiation shone on it)?
Thanks, Lewis

 

[berkeman]

Hi, I am doing the planning exercise aswell.

Would a voltmeter be ok to measure the output of the photocell because voltage is proportional to resistance ( which varies with amount of infrared radiation shone on it)?
Thanks, Lewis

No, you can't use a voltmeter directly. You can put an explicit resistor in series with the photodiode, and measure the voltage drop across the resistor from the photocurrent.

Check the accuracy and sensitivity specs for your DVM, and check the specs for the output photocurrent that you expect from your photodiode/photocell. Then figure out which method will give you the best data...

 

[lewmang]

thanks for the quick reply.
Ok, so ill use a fixed resistor in series with the LDR, would an LDR pick up infrared radiation?
And what kind of voltage would be needed to put through the currents.
By the way i don't hav to actually do the experiment i have to design it with some theory as evidence for my actions.

 

[lewmang]

*wht kind of voltage would be needed to send through the circuits

 

[berkeman]

I didn't say use a resistor in series with a Light Dependent Resistor. As for the IR sensitivity of LDRs, I have no idea. As part of your project, you need to find and read the datasheets for the various light sensing component technologies, and list the advantages and disadvantages of each. That will guide your choice as to which one to use for the IR sensor experiment.

 

[lewmang]

what do you mean by datasheets? We havn't been given any data to use it is basically start from scratch

 

[berkeman]

what do you mean by datasheets? We havn't been given any data to use it is basically start from scratch

Use the Internet. Use google some, look for companies that manufacture the components that you are considering, download the PDF datasheets for the components, and start reading the specifications. That's how it works in the real world after school...

Like, I just googled +photodiode +IR and got lots of hits. Here's the first one:

http://photodiode.globalspec.com/Industrial-Directory/photodiode_ir

 

[lewmang]

I read earlier on in the thread that you said maybe use a small balb, would this be sufficient too?

 

[lewmang]

what kind of photocell would be needed to measure infrared radiation that would be available in schools?

 

[berkeman]

What do you mean by "available"? You mean like what can you take apart to pull out a photocell for experiments (like an elevator door sensor, or somebody's camera light sensor)? Won't you get in trouble if you start taking things apart at school? Or do you mean some part that is easy to order from a place like Digikey.com?

 

[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.

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