Phototransistor switching freqeuncy

Discussion in 'General Electronics Chat' started by Alasttt, Jan 9, 2016.

  1. Alasttt

    Thread Starter Member

    May 13, 2015
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    Hi,

    I have an infrared emitting LED that shines onto a phototransistor. I am inputting a square wave into the LED and looking at the output of the phototransistor. At a low frequency the output is correct. However my data is 4khz, and at this frequency the output of the phototransistor is nothing like a square wave. See the pic attached for input data and output of the phototransistor.
    I also noticed that the frequency of the output wave is only 2khz, so I think its something to do with the phototransistor not being able to switch its output state quick enough.



    Any thoughts ?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Post a schematic.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What type (brand and model) of scope are you using? What probes are you using?
    This matters because some cheap USB oscopes are really low frequency and you get an effect called "alias-ing". Essentially, destructive interference of input signals and sampling frequency of your scope.

    Also, post a schematic. I know it seems like it shouldn't matter but it can if your resistor values are too high.

    A photo transistor can easily handle 40 kHz.
     
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  4. Papabravo

    Expert

    Feb 24, 2006
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    Looks like your output circuit has one driven state and one floating state. This suggests a missing resistor. A schematic would be most helpful. It is the lingua franca of electronics.
     
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  5. Alasttt

    Thread Starter Member

    May 13, 2015
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    I have used two oscilloscopes, a picoscope 3205 and a tektronix tds 210. I should also add that I am using an infrared emitter and receiver, not a normal LED with visible light. The circuit works at low frequency, I have attached the schematic and the datasheet for the receiver. It is an SY-32PT

    Its a simple circuit, apologies for the phototransistor image as the schematic software I used did not have this component so I had to use an image.

    Hope this helps
     
  6. dl324

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    Are you certain the schematic reflects how you have the photo transistor connected?
     
  7. Alasttt

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    May 13, 2015
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    Yes. The flat edge pin goes to 5v via a 5.6k resistor. and the other pin goes directly to 0v.
     
  8. dl324

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    Then you have it connected backwards.
     
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  9. Alasttt

    Thread Starter Member

    May 13, 2015
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    Hi,

    thank you, reversing the transistor and still taking the output from the emitter gives more or less the correct output you can make out the highs and lows. But it is horrendously noisy. See attached, might a capacitor sort this ? zoom.PNG
     
  10. GopherT

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    Nov 23, 2012
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    Yes, as DL324 pointed out, swap the collector and emitter. On the other hand, your drawing is best corrected by moving the ground symbol to the emitter and the supply + resistor to the top (collector pin in your drawing). With those changes, you now have a more "standard" format for a schematic. That is, more positive to the top of the drawing.
     
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  11. GopherT

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    No, no. Now that the transistor is flipped, you need to move the resistor between + supply and collector. Take the signal from the collector of the transistor. I hope that helps.
     
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  12. Alasttt

    Thread Starter Member

    May 13, 2015
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    I have connected collector to + via resistor. and the emitter directly to ground. Taking output from the collector just gives a straight 5v line. Output from the emitter gives something like the expected output. but its very noisy
     
  13. GopherT

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    Post your new schematic. And double check that your new schematic matches your breadboard.
     
  14. Alasttt

    Thread Starter Member

    May 13, 2015
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    Data.PNG
    Thats the input to the infrared emitter, and what I would like to see at the output of the phototransistor.
    line.PNG
    This is the output at the collector.
    em.PNG
    This is output taken from the emitter
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    First, change R1 on the photo-transistor from 5.6k at collector to 47k or 100k. Make sure your probe is at the collector and the clip on the probe is connected to the emitter of the phototransistor (which is connected to ground).
     
  16. Alasttt

    Thread Starter Member

    May 13, 2015
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    schematic below


    newsch.PNG
     
  17. GopherT

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    @Alasttt

    See post 15 above and make sure your IR source is well aligned with the photo transistor. Some have only a 10 degree spread of the light. Likewise, the phototransistors have a specific focal angle too. It is easier if you separate them by 3 or 4 inches and inspect for co-linearity.
     
  18. Alasttt

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    May 13, 2015
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    I have changed it to 910k, the closest I had. The probe has a + and - clip. - clip is connected to 0, +clip is connected between the resistor and the collector. the output is still a straight 5v line.
     
  19. GopherT

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    910k is too much and signal impendence is so high everything falls to noise.
     
  20. Alasttt

    Thread Starter Member

    May 13, 2015
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    Sorry its not a straight line it looks like below phototransistor output.PNG
     
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