Simple transistor-op amp interface advice

Discussion in 'The Projects Forum' started by hsherwood68, Oct 29, 2013.

  1. hsherwood68

    Thread Starter New Member

    Oct 29, 2013
    4
    0
    Hello kind people! I want to represent the light falling on a photodiode as the voltage across a resistor and then amplify that voltage such that the minimum is between 0-1V and the maximum is between 4-5V. This then gets fed into an ADC.

    http://harryysherwood.files.wordpress.com/2013/09/schemeit-project.png

    The individual blocks of the above circuit appear to work fine, but my problem has been connecting the first (transistor) to the second (741 op-amp). Does anybody have any advice for how these two should be connected so that I can achieve what I need to? I am also aware that there may be other mistakes in how I've wired up my op-amp, so do let me know if the middle bit needs re-designing!

    Thank you so much!
     
    Last edited: Oct 29, 2013
  2. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    You can't feed the input of a 741 with any voltage that is closer than 3 volts to the power supply voltage. A 741 with a 5V supply is useless. Pick a different amplifier chip or give the 741 +/- 15 volts to work with.

    Besides that, the voltage at R4 is never going to cross the 5 volt input. This circuit is just wrong. First guess? Make an adjustable threshold voltage for the non-inverting input.

    Partial answer. Have to go to work now.
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    Do yourself a favor: look up <photodiode transimpedance amplifier> and work from the designs you find. And yes, select an amplifier that works on the voltage you intend to use!
     
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