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light sensor advice

Discussion in 'Sensors and Actuators' started by mfiny, Jul 8, 2010.

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  1. mfiny


    Jul 4, 2010
    I need an IR sensor that can sense IR light intensity. Essentially what i want to be able to do is get an IR light source (say some led's or a lamp) and be able to have the voltage reading from the sensor vary as the light source moves closer or farther away from the sensor. I am hoping to use a distance of between 0 - 0.5m.

    I have tried constructing an IR light sensor using a phototransitor and a resistor connected to a voltage source (as per a previous post), however this is no good as it is only usable at distances between 0 and ~15cm.

    Can anyone suggest what i might need to do? is there an off the shelf item that can achieve this? (that is not to expensive)
    If not then what should i be using to construct this circuit?

    Thanks for your help
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You need to get an IR photodiode and ensure it receives only IR light (typically IR diodes in dark cases do this -- if it's in a clear case you'll need an IR filter).

    Connect this to an amplifier (say an op-amp) with appropriate gain so that the range of intensities you require cover a useful range of voltages.

    There are some special photodiodes designed for light level measurements. They typically have a very large surface area so they can catch as many photons as possible.

    an example of such a diode is here. Note that this example is sensitive over the entire visible spectrum, but also well into the near infra-red. With an appropriate filter it may be the sort of device you'd use.
  3. mfiny


    Jul 4, 2010
    Thank you Steve.

    With regards to the actual circuit, would it suffice if i just had the photodiode in series with a resistor connected to a voltage supply (as per my old setup), but then use an op amp to amplify the voltage that i will be measuring?

    Is there any specific properties i should be looking for with the photodiode (as in with regards to specifications)?
  4. Laplace


    Apr 4, 2010
    Except for applications requiring sub-microsecond response times, I tend to think of a phototransistor as just a photodiode with gain. You can use both with the same type of transimpedance amplifier. It should be easier to find a photodiode with a larger aperture whereas a phototransistor will likely come with an integral focusing lens further increasing its sensitivity. It may be easier to increase the effective aperture of the photodiode with optical gain, i.e., adding a larger focusing lens. It is not clear that your application would favor one over the other.

    I have been working on a project using PIN photodiodes and once ordered the 'filtered' version of the BPW34, not realizing at the time what that meant. The BPW34 diode comes embedded in transparent plastic with full spectrum sensitivity whereas the BWP34FA comes embedded in black plastic which is transparent only to infrared light.
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