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
    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.

    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


    Nov 30, 2010
    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
    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!