Comparing Photodiode and Phototransistor characteristics

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

  1. Sensacell

    Thread Starter Senior Member

    Jun 19, 2012
    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


    Apr 5, 2008
  3. Sensacell

    Thread Starter Senior Member

    Jun 19, 2012
  4. takao21203

    AAC Fanatic!

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

    AAC Fanatic!

    Oct 27, 2012
    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 Senior Member

    Jun 19, 2012
    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
    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.