Comparing Photodiode and Phototransistor characteristics

Discussion in 'General Electronics Chat' started by Sensacell, Sep 18, 2014.

  1. Sensacell

    Thread Starter Well-Known Member

    Jun 19, 2012
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    I am working on a product requiring a photo sensor to detect white light, I need the largest possible dynamic range (1000:1 light levels) at the minimum cost.

    Response time: 200 us rise/fall
    Photo-conductive operation- (needs to sink current in proportion to light levels)
    Long term / temp stability not required, device will autocal for every measurement.
    No need for good linearity, I am not building a measuring device, just a lights on/off detector, with a large range of potential light intensities.

    Looking at the range of options, phototransistors seem to typically have a poor ratio of dark current to light current, but tend to be cheaper.

    Photodiodes seem to have a better ratio of dark/light currents and are fast, but the parts are MUCH more expensive.

    Are there any tricks to getting the wide dynamic range I seek? As cheap as possible?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Sensacell

    Thread Starter Well-Known Member

    Jun 19, 2012
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  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    BPW34 is not that expensive
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Photodiodes (can be) very fast but are not too sensitive - Phototransistors have gain which makes them more sensitive, but they're slower.

    Cd-S LDRs are the easiest to interface, and are the closest match to the human eye response curve - but they're *REALLY* slow, and I'm not too sure about RoHS compliance (Cadmium is *VERY* toxic!).
     
  6. Sensacell

    Thread Starter Well-Known Member

    Jun 19, 2012
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    Thanks all for your feedback, I will be experimenting with a bunch of sample parts, both photodiodes and transistors.
    I have a feeling the photodiode is going to be the winner.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You don't state the light levels, but even so you should be able to develop a signal from 10 mv to 10 volts with a photo diode easily enough.
     
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