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Little Problem with phototransistor

  1. Oct 25, 2009 #1
    Little Problem with phototransistor !!!

    Hi Forum ,i am using a laser module, phototransistor SFH309-5, and 74HC14N (Circuit in the attachment) to detect the speed of the motor. On the Vout2 i measured frequency up to 2,8KHz when the speed of the motor is 28 000 RPM (because 6 reflected beam every rotation). But when i increase the motor to 32 000 RPM the frequency on Vout2 is 500Hz , and it should be 3,2KHz , what can be the problem ?
    Thank you in advance ,
     

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  3. Oct 25, 2009 #2

    uart

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    Re: Little Problem with phototransistor !!!

    You're missing pulses (pulses are joining together) because either the pulse width from the mirrors is too narrow or the photo transistor is too slow (possibly due to saturation) or a combination of both.

    You need to look at the waveform on pin 1 (74HC14) to determine exactly which is the case.
     
  4. Oct 25, 2009 #3
    Re: Little Problem with phototransistor !!!

    Hello uart , i tried to measure the pulses on Pin 1 of the 74HC14 , i read a 3,2KHz and 1,1Volt ,when the speed of the motor is 32 000RPM , which is true i think that 74HC14 needs near 3,3V to work properely , i think to increase the valu2 of R2 is a good idea? do you recommand it to me ?
    Have a nice day,
    Anita
     
    Last edited: Oct 25, 2009
  5. Oct 25, 2009 #4
    Re: Little Problem with phototransistor !!!

    I would definitely increase the value of R2 as 1.1V probably isn't enough for your uC to detect. I would bump it up to at least 10kOhm and get a minimal voltage of 1.8V.
     
  6. Oct 25, 2009 #5

    uart

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    Re: Little Problem with phototransistor !!!

    You were already told this last week in your thread regarding the same circuit.

    You'll need to post a sketch (or photo etc) of the actual waveform at pin 1 if you want a definitive dignosis of what's wrong.
     
  7. Oct 25, 2009 #6
    Re: Little Problem with phototransistor !!!

    If R2 is too high, the RC time constant with distributed capacitance in coax cables, etc. may be too high. You need to discharge the cable in less than 10 microseconds.
    Bob S
     
  8. Oct 25, 2009 #7

    vk6kro

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    Re: Little Problem with phototransistor !!!

    Even at its best this circuit is only just managing to work because you need almost the full supply voltage as output just to drive the 74C14.

    There is a chip that might help. It is the 74C04 which has the same pinout as the one you are using but it has inverters in it that can be used as amplifiers.

    74C04 amplifier.PNG

    As an amplifier, you can treat it like an opamp, so the gain is equal to the ratio of the feedback resistor to the input resistor.

    You only need a gain of 2 or 3 to get a square wave output that is equal in height to the supply rails.

    You would have to experiment with the resistor values, but I would start with 10 K as the feedback resistor and 3.3 K as the input resistor. Definitely use a 5 volt supply.

    OR, you could just get a LM324 quad opamp and wire up one of the opamps to give a gain of 2, 3, 4 or 5 and see how you go after that. This is only amplifying 3 KHz, so it is not a difficult job.
     
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