Help! Strange Opto listener

Discussion in 'The Projects Forum' started by ChrisChemist116, Jun 5, 2009.

  1. ChrisChemist116

    Thread Starter Well-Known Member

    Mar 13, 2009
    I have been working on this project at the request of some of my classmates at the university and is called the Opto listener.

    The device is intended to "listen" light via photodiode. We study chemistry and we thought this project could be great for our science expo on july.

    In order to make the Optolistener i took some ideas from tom petruzzellis book ( The design is very simple and can be seen on the attached file (sorry for the image quality, i didn't had time to make the schematic on the computer).

    Based on instructions in the book and the schematic i managed to make the device to work, but the problem is, the optolistener doesn't work if the photodiode's cathode is connected to 9V supply anode as stated on the schematic.
    What can be the problem?. Bad design?.
    Any ideas?. Help and assistance will be appreciated.
  2. Audioguru


    Dec 20, 2007
    Your schematic is wrong:
    1) Pin 3 input of U1 is connected to 0V instead of being biased at half the supply voltage so the 741 opamp will not work.
    2) The signal from the cathode of the photo-diode is shorted to the positive supply instead of being fed through a high value resistor from the positive supply so it will not produce a signal.
    3) The inportant 0.05uF capacitor in series with a 10 ohm resistor from the output to ground of U2 are missing (as shown in its datasheet) so it will oscillate at a high frequency and will not amplify sounds.
  3. ChrisChemist116

    Thread Starter Well-Known Member

    Mar 13, 2009
    Thanks for your patience on pointing out the mistakes on the circuit for me. I checked the schematic these days and i also came to that conclusion regarding the issues you wrote. On the meantime i spend time reading datasheet for LM386 and i will fix the project. As long i can find time this week i will post the results.
  4. RussellP635

    New Member

    Sep 12, 2016

    I asked an electronics engineer to build me one of these circuits (the opto listener) years ago , and he said that he had to redesign it. I know this is from a long time ago, but did you keep a copy of the redesined circuit? I am trying to modify it so that I can have a digital readout of the frequency.


  5. hp1729

    Well-Known Member

    Nov 23, 2015
    Design 844 optolistener.PNG
    View attachment 111902
    Something like this? The FPT100 is a photo transistor and needs the added resistor, R3. For a color sensitive design you could use an LED as the opto sensor and omit R3. An LED will generate voltage when exposed to light. The colored lens gives you filtering. Forward bias the LED, cathode to ground.
    Some newer op amp than the LM741 would be an improvement. LM358 or newer.
    AD820 or such. Watch the power requirement on the op amp you choose. The killer is that the LM741 does not work well within a couple of volts of the power rails. The input stage design of the LM358 is better for this.
    Last edited: Sep 13, 2016