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PhotoTransistor and Hex Schmitt trigger Inverter

  1. Oct 11, 2009 #1
    Hello forum , i would like to ask you about this circuit in the attachment , i don't have any voltage on Vout , could you tell me where is my mistake.
    Thank you in advance,:cool:

    Attached Files:

  2. jcsd
  3. Oct 11, 2009 #2
    What voltages are you getting on the emitter, when turned on?

    Low Vcc on the Schmitt trigger minimizes the trigger window. You could be in a constant on or off state, and because of the sensitivity of the photodetector, it's unable to bring the transistor to a full low state or high state depending on your configuration.
  4. Oct 11, 2009 #3
    Hi, thank you for your reply,
    the voltage is ~240mV,what do you recommand me ?

    Vcc=3.3V it's the ideal voltage for the schmitt trigger refering to the datasheet
    Have a nice day,
  5. Oct 11, 2009 #4


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    http://en.wikipedia.org/wiki/Schmitt_trigger" [Broken]
    Last edited by a moderator: May 4, 2017
  6. Oct 11, 2009 #5
    What are the threshold levels for this chip?

    I checked a datasheet real quick, may or may not be what you have:

    Vcc = 4.5v
    V+ = 2.38v
    V- = 1.4v
  7. Oct 12, 2009 #6


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    Across R2, you should get 0 Volts when there is no light and 3 volts when there is light.

    R2 is 1200 ohms and the current through it should be 2.5mA.
    So, V = 1200 ohms * 0.0025 Amps = 3 volts.

    If you are only getting 240 mV across R2 when it is lit (and presumably 0 volts when it is dark) then you don't have enough light getting to the phototransistor.

    This would be why your Schmitt trigger isn't giving any output.
  8. Oct 23, 2009 #7
    Hello , thanks for your reply , it is true that not enought light , i change the system and now i have across R2 2.2 Volt , is it enought for the schmitt to work properelly?
    Thank you in advance ,
  9. Oct 23, 2009 #8


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    1. Increase R2 until you do get enough voltage.

    2. You might be a bit better off with Vcc=5 volts
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