Hi, new to the forum. I'm doing AS level Physics and I need some guidance with my practical plan. It is about automatic doors using infra-red radiation which is detected by a photo detector. I understand the basic principle and how they work but I haven't really an idea on how to set this up in the classroom.

We are investigating how the output from a photocell depends on its distance from a point source of infra-red radiation.

We need:

a) the procedure to be followed
how the output of the photocell would be measured
c) the range of distances that would be used
d) the ranges of any measuring instruments that would be used
e) relevant circuit diagrams
f) safety procedures
g) how to ensure accuracy and reliability

Any ideas? Thanks in advance guys.

i have been given the same task to do, and also am unsure how to approach the problem.

i know you need to start with a fixed infra-red source and a ajustable photo cell (some sort of light detector)

thats as far as ive got with it so far

hi im samee
im also having alot of difficulty with this planning exercise and also need some help.
please somebody help me
thanks

anyone got any ideas??
thanks

Some suggestions...I take it this is a LIVE experiment and that you will actually be doing it in class with real components?

The answer i.e. where do you start from depends primarily on what components (IR photocell and possibly an IR radiation source). These items should be available from Radio Shack or wherever, and should have technical specification sheets so that you can see what ranges they operate under.

IR radiation is naturally ocurring, and if your detector is sensitive enough, you should be able to use it PASSIVELY (without providing IR illumination, using background IR only). In most cases, the Human body is quite hot compared to background unless you live in the desert and your building is not air conditioned (in which case I recommend you move).

OH Wait I see, you want to use the IR source moving to generate a signal on the photo cell.

here is an IR matched source and detector pair from Radio shack
http://www.radioshack.com/product/index.js...rentPage=family

1 x infrared emitter - 2V 40mA
1 x phototransistor detector - 20V 25mA

so you need a 2v source for the emitter and a current limiting resistor, (like a couple AA or AAA batteries in series 1.5 x 2 = 3v)
and a up to 20 volt (12 volts would probably be easiest) source for the detector (like a lantern battery, you again need current limiting or bias resistors), you measure voltage drop or current flow across the transistor (emitter to collector, as the Base is the photo input)

It depends on the transistor whether more light = more current / higher voltage, or the other way around. (NPN or PNP junction I think)

The maximum voltage of the power source determines the range needed on your meter
The operating characteristics of the emitter/detector determine the distances needed, I recommend 10 feet probably then successive halves of the distance to an inch or so. That should get you min to max reading I would think.

Hope that helps you

holy jesus wats so hard about it?!?!

infra red LED and a LDR....closer u move the LED to LDR greater the light intesnsity is and therefore the lower the resistance around the LDR is (measure using a micrometer). bobs ur uncle u have relationship between distance and resistance.

I would also recommend using a tube to prevent as much natural light as possible shining on the LDR while the LED is being used.

on a lighter note how much does stadium arcadium rock