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Need help with a sensor circuit using a photodetector

  1. Feb 15, 2007 #1
    hello, im currently working on a simple school project to make a sensor circuit. i need the circuit to include a photodetector (either photodiode, phototransistor or LDR) to sense and measure the intensity of a typical laser beam (from a cheap laser pointer), the main aim is to get a different reading depending when the distance of the laser from the photodetector is varied. so im seeking advice on which photodetector is the most suitable for this purpose, i.e. be sensitive enough to detect the difference in the intensity of the laser when it's distance from the photodetector is simply varied. if this is not possible at all, then my other option is to also vary the intensity of the laser itself.

    thanks, please give reasoning aswell with your advice
     
  2. jcsd
  3. Feb 15, 2007 #2

    berkeman

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    Well, since a reasonably good laser beam doesn't diverge very much, you're going to have to vary the distance quite a bit. Start by measuring the spot size versus distance -- how far away does spot finally get out to 1cm, for example?

    The only other thing that attenuates the beam intensity with distance is scattering off of dust particles and such in the air, and again, you have to go through a lot of air to get much attenuation.

    Probably a better-controlled experiment would be to use two polarizers, and plot the attenuation of the beam with respect to the angle of the two polarizers. Are you familiar with this type of experiment?

    As for the best detector, you would want something that has a reasonable detection area (a few mm^2), over which the output current (or whatever) does not vary much. Most photodiode detectors that I'm familiar with have a definite "sweet spot" for a focused beam to hit, so they probably wouldn't be consistent enough for use in your laser beam detection experiement.

    What can you tell us about the characteristics of the three detectors that you've listed? What are they made of, and what kind of external circuit would you use to get data from them?
     
  4. Feb 17, 2007 #3
    basically its sort of a proximity sensor circuit, using a potential divider, the voltage reading should change with the distance between the laser and the photodetector. i have to complete a full plan before being able to experiment myself, so i need to know which photodetector would sense a difference in the intensity of the laser if it is moved a few cm closer to it or further away. if this is not possible because of the strenght of the laser beam, then do you know anyway i could achieve the same goal using a normal light beam or infra red perhaps. the problem with these 2 is i need to make the light or infrared more directional so it hits the photodetector only if they are in line with each other.

    as for the characteristics of the 3 photodetectors,i dont even have them yet, i have to decide which ones to get thats why im asking for help, they can be anything as long as they are not expensive so preferably a CdS photocell or some sort of photodiode or even a phototransistor.

    lastly i cant use any expensive components, thanks
     
  5. Feb 17, 2007 #4

    AlephZero

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    You can put a diverging lens in front of the laser, to make the spot size vary the way you want. It will also make the device safer, if you look straight at the beam!

    Making the beam larger than the detector, at the working distance, also removes the "detector sweet spot" issue.
     
  6. Feb 21, 2007 #5
    any more ideas?

    preferably using electronics and circuit components rather than optical components

    thanks
     
  7. Feb 21, 2007 #6

    berkeman

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    Staff: Mentor

    I think AlephZero's idea of using a diverging lens is your only bet for getting a variation in intensity over the small distance changes that you have mentioned.
     
  8. Mar 10, 2007 #7
    photocell

    how can i calculate the output of the photocell

    plzz reply as soon as
     
    Last edited: Mar 10, 2007
  9. Mar 10, 2007 #8
    how can i calculate the output of the photocell. i have used a filament lamp and prism which illuminates the spectrum onto a white screen. The photocell is moved in between the prism and white screen at different lengths such as 2,4,6,8,10cm from the prism. and what reading is accurate from the milliammeter such as 3sf or whatever the readings come up as. Is the reading from the milliammeter the output of the photocell or is the intensity from various distances between the filament lamp power output and the sphere of the photocell

    plzz reply as soon as
     
  10. Mar 10, 2007 #9

    dlgoff

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    berkeman might want to move your question to another thread since your question is a little different the the OPs. But I'll give you some considerations when measuring the output from the cell with your setup. First, the photcell doesn't see all colors equally. And since you're using a prism your photocell may be seeing different colors as the distance from it increases, and will dependon the prisms orientation. The manufacturers specs will probably have a graph of the ouput vs. the lights frequency. Typically in applications one trys to find a source thats frequecy is around the sensors peak frequency inorder to get the best response. Second, the light source is a filament bulb and doesn't emit all the colors you're seeing at the same intensity.

    Regards
     
  11. Mar 11, 2007 #10
    what other sort of light source do u suggest i should use. Does the filament lamp emit infrared rays. Is using the prism a good source for reflecting the light rays. im only looking for infrared light in which the photocell does sense infrared light just i think. Is this true? Relating to my question how can i measure the output from the photocell. or is it the reading from the milliammeter.

    reply as soon as

    thanks
     
  12. Mar 12, 2007 #11

    dlgoff

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    A prism refracts the light; refracting the higher frequency light at a grater angle than the lower ones. That's why you see a "rainbow" of colors. Maybe with a light bulb you wouldn't need the prism if you are just trying to get an output with varying distances from the source like the OP is trying to do. If your photocell is sensivity to IR (and it probably is) and since filament light bulbs emits lots of IR (but also visible light) then you will get an output.

    Measuring the output with a milliamp meter will work okay. You would want to record the value of current at various distances.

    Hope this helps
     
  13. Mar 13, 2007 #12
    so the milliamp is the output detector by the photocell. Is there any calculations i need such as the intensity of light from the filament lamp that lands on a sphere which is the photocell like what hoot said about explaining the light intensity cause im wandering why i need that as im only investigating the output produced from the light.

    Also how accuate should the reading be on the milliammeter eg 3sf

    reply as soon as

    thanks
     
  14. Mar 13, 2007 #13

    dlgoff

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    Inorder to calibrate your photocell you would need some standard light source or another calibrated lightmeter. You would need to know what intensity is falling on the cell; usually measured in candela per square metre (cd/m2). Now I think in your case you won't need to go to all the trouble and expense. So just use your "light meter" to give relative differences with varying distances from the source. That's going to be about the best can do. If you want to learn more about light measurments you might want to start with this: http://en.wikipedia.org/wiki/Light_meter

    Hope this helps.

    Oh. "Also how accuate should the reading be on the milliammeter eg 3sf" Test you lightmeter and see if you really need 3sf. Maybe one or two will do.
     
    Last edited: Mar 13, 2007
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