Designing a photo detector circuit

Discussion in 'General Electronics Chat' started by shriks, Apr 14, 2008.

  1. shriks

    Thread Starter New Member

    Apr 14, 2008
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    Hi,

    I am currently designing a photo detector circuit which would give out a TTL pulse in response to a black or white image on a screen changing at 30Hz. Since I am an amateur at electronics I have been looking on the internet for ideas, and came across this particular circuit

    http://www.emant.com/324003.page

    But I am not able to decide if this could be of use to me. And also if the circuit is this simple.

    Could anyone advice me on this ? Any suggestions on other possible circuits/ideas is also appreciated.

    Thanks in advance.

    Shriks
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The divider circuit at the top of the page will work. You can send the output to a single source comparator (maybe an LM393) to generate a TTL compatible signal. Using a potentiometer for second comparator input will allow adjustment.
     
  3. shriks

    Thread Starter New Member

    Apr 14, 2008
    8
    0
    Hi,

    I finally managed to get the photo detector working. I now need to transform the output in terms of a TTL signal. My input is either black or white color. For which the photo dector has an ouput of ~2mV(for black) and ~22mV(for white).
    Can anyone suggest a suitable TTL ciruit that I can combine with my photodetector circuit ? Or can I modify the existing circuit to get a TTL ouptut ?

    Thanks in advance
    shriks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Thingmaker3's suggestion of using a single comparator will do that for you.

    Have one input of the comparator set to 12mV using a resistive divider network.
    place your photo detector output on the other comparator input.

    The output of the comparator may be open-collector. In that case, you will need to use a 1k Ohm resistor between it and 5v to get a TTL-level signal output.
     
  5. shriks

    Thread Starter New Member

    Apr 14, 2008
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    0
    Hi SgtWooki,

    I finally tried the circuit for a TTL output as suggested. But I only getting a constant voltage of 2.35V for both white and black conditions from the photo detector.
    The output from the photo detector is now 0.8V(for black on screen) and 5.5V(for white on screen).
    For the TTL output I am using LM293 comparator with Vcc=5V. One input(pin 2) to the comparator is from the Photodetector and the other input(pin 3) is a reference voltage of 2 volts (using a voltage divider). The output is taken across a 1K resistor between the Vcc and ouput(pin 1).

    Is the above circuit OK or is there any thing missing ?
    Alternately, cant I directly get a TTL using the photo detector(BPW 34) ?

    Kindly advice.

    Thank you,
    shriks
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Can you post up a schematic of your circuit? That would be very helpful.
     
  7. shriks

    Thread Starter New Member

    Apr 14, 2008
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    0
    I have attached the crude circuit diagram.
    The input to the BPW34 is from a video with either black or white squares at 60hz. For which I get an output of ~0.8V(when black) or ~5.5V(when white).
    But at the output of the LM293 I get a constant 2.2V instead of a TTL signal with respect to the black/white.

    Any suggestions on what could be wrong ?

    Thanks in advance.
    Shriks
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 34M resistor at the input of the LM293 is far too high. Its input bias current is sourcing (going positive) and causes a max low input voltage of 8.5V (higher when the leakage current of the photodiode is included).
    Maybe your voltmeter or 'scope forced the voltage down.

    The output voltage from a comparator is supposed to be either close to 0V or the positive supply, so yours is probably oscillating as described in its datasheet because your circuit does not have hysteresis.
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    A resistor in the order of 10K (maybe less) could give the results you want. For reliability, the current through the photo diode resistor should be at least 20 times greater than the maximum bias current through the input of the comparator.

    I would suggest you to use a photo transistor instead of a photo diode, since the former is far more sensitive. The photo diode may not give the results you want.
     
  10. gee_emm

    Active Member

    Jan 16, 2008
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    0
    In making a "light meter" circuit a bit ago, I found that photo-diodes actually put out current, not voltage. I used an op amp current-to-voltage converter, then ran it to my comparator.
     
  11. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    Why not use an op-amp to amplify the signal then use a comparator to level shift?

    I am off to work, but look up photovalic or photoconductive mode with photodiodes.
     
  12. shriks

    Thread Starter New Member

    Apr 14, 2008
    8
    0
    Thanks all for the suggestions.

    But I would like to use the same BPW34 and continue as I am short on time and have complete this project as soon as possible and so have no time to experiment with new photo sensors.

    I searched around the internet and found a comparator circuit with Hysterysis(attached to the post). Can anyone advice on how I could modify this circuit for my purpose?

    I understand that Vreference would be half of the voltage across the photo diode resistor load. And for a TTL signal the Vcc = 5V.

    Thanks in advance.
    shriks
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I would suggest you consider using a two transistor current mirror to interface your photodiode to your comparator circuit. The advantage to a current mirror in this application is that you would be able to maintain a constant reverse voltage across the photodiode of 5 volts as indicated in the graph that appears in the datasheet for the photodiode. The photodiode would be installed in the place of R(bias) in the example two-transistor current mirror in the link I have provided. You would then be able to size the resistor R(load) to provide the signal swing you needed for the input to the comparator. The improvement in the response of your photodiode would be a bonus in this case.

    hgmjr
     
  14. shriks

    Thread Starter New Member

    Apr 14, 2008
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    0
    Thanks alot fór the suggestion. I will try it out. But to which comparator circuit should I combine the mirror circuit with, the first circuit that I had posted or the second comparator circuit with hysterysis. And is the comparator circuit OK or does it need and modifications ?

    Thank you
    shriks
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The circuit you've drawn is a Schmitt Trigger. You might be able to get that in a TTL or other logic family chip.
     
  16. shriks

    Thread Starter New Member

    Apr 14, 2008
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    If I understood you correctly you are saying the Circuit with hysterysis should work fine ?
     
  17. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    The comparator with hysteresis should work fine (your second circuit). With the current mirror circuit, you should be able to select the value of R(load) that will give you a signal swing that transitions virtually the entire range of your power supply.

    One thing you may encounter is that ambient light will have a tendency to register on your photodiode due to the infrared light that it contains. Depending on how much exposure to ambient light your photodiode receives, you may not have a problem. Just be aware that many light sources contain varying amounts of IR light.

    You might go ahead and sketch the circuit that you plan to implement showing the current-mirror and the comparator as whole. There is likely to be some component value tweaking needed to get things fine tuned for operation.

    hgmjr
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    Any progress update on your detector project?

    hgmjr
     
  19. shriks

    Thread Starter New Member

    Apr 14, 2008
    8
    0
    HI,

    The work on the photo detetor is on a pause. I will resume the work from the end of this week.

    Will keep you updated.
    Thank you.
    Shriks
     
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