photodiode circuit: NEED HELP

Discussion in 'The Projects Forum' started by pougetp, Jul 12, 2008.

  1. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    Hi,

    I'm trying to make a circuit to detect a white flash on screen tV.. and to send a TTL for each flash (60Hz monitor).
    Attached the circuit I'm working on... not working :mad:.... I spent the last two days... I do not know what is wrong with it.

    I really need help there....:eek:

    Thanks a lot

    pierre
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you make a schematic in stead of a wiring diagram.
    The picture may be some bit larger, this one is very hard to read.

    Greetings,
    Bertus
     
  3. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    I did have a proper -5/0/+5v input. I checked it and this part is working fine.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What happens on pin 12 of the UA747?
    What is the voltage in this pin when there is light and no light?

    Greetings,
    Bertus
     
  5. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    Ok Bertus,

    I'm preparing the system and will do the measurement.
    So far the 123N is hot when we plug the circuit.
    We REALLY appreciate your help.
    I'm sending the measure in a minute...

    pierre
     
  6. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    The voltage is 4.5v and do not change in/out light.
    We tested the photosensor is working fine we did test it.
    Does the diagram sounds reasonable?

    pierre
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    Take th 75K resistor from pin 12 and put it to the - 5 Volt.
    Do the test again.

    Now I see it. The 74LS123 may not have negativ voltages on an input.
    You have - 5V at pin 3, this can destroy the 123.
    Put this pin on ground.

    Greetings,
    Bertus
     
    Last edited: Jul 12, 2008
  8. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    Hi Bertus,

    ok we put the pin 3 at ground...:( of the 74LS123
    Still not working...
    The voltage is at the UA747 is 4.5v. The 74LS123 is not warm anymore :).
    We are ok to start from scratch if you think this scheme is simply wrong....

    pierre
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Take the 75K resistor from pin 12 and put it to the - 5 Volt.
    Do the test again.

    Greetings,
    Bertus
     
  10. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    ok the 75k is at the -5 now.
    no difference.. would you like a measure?
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes, measure with and without light on pin 12.
    Also measure with and without light on pin 1.

    Greetings,
    Bertus
     
  12. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    :confused:....
    A diagram... please help. I really need this project by next week.


    :mad: should have done me electronic.

    Thanks you all in advance

    pierre
     
  13. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    pin 1 and 12 of the 747 or the 123 ?
    Probably obvious for you....:p
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    We are looking at the phototransistor, so look at the 747.

    Greetings,
    Bertus
     
  15. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    on the 747:
    pin 12: -3.25v
    pin 1: +5v

    no change with light.

    pierre
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    It looks like you phototransistor does not react at the moment.

    How did you measrue with and without light?
    Did you cover the phototransistor with something for the dark measurement?
    Did you shine with a light on it for the light measurement?

    Greetings,
    Bertus
     
  17. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    :)
    ok I did put the diode in darkness.
    Now pin 1: is changing from +5.. 2v
    But pin 12 is not changing at all: 3.25v

    Thanks again Bertus for all your effort... I really hope we can fix it

    pierre
     
  18. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Take the potentiometer from pin 7 to pin 2 (take the ground off).
    Take the potentiometer from ground and put it to + 5 Volt.
    Now you can put pin 2 to about 4 Volts with the potentiometer.
    Do the light and dark measurements again on pin 1 and 12 of the 747.

    Greetings,
    Bertus
     
  19. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    *
    *Take the potentiometer from pin 7 to pin 2 (take the ground off).

    we've got 0.2v between pin 7 and 2 after taking ground off (from pin 2)

    *Take the potentiometer from ground and put it to + 5 Volt.

    ok we took the ground of the potentiometer off
    and put it to the +5v

    *Now you can put pin 2 to about 4 Volts with the potentiometer.
    ok we put 4volt between pin 2 and 7 by adjusting the potentiometer.
    The potentiometer is still connected to pin 7, +5 and -5

    *Do the light and dark measurements again on pin 1 and 12 of the 747.
    now pin 1 is 5v but no change with light
    and pin 12 is 3.25 with no change with light.


    pierre
     
  20. pougetp

    Thread Starter Member

    Jul 12, 2008
    27
    0
    after more measures
    and a disconnected wire:mad:
    pin 1 is varying from 1.25 to 3.5 dark to light
    pin 12 is very varying but very small change from 3.25 to 3.18 with very bright light.

    pierre
     
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