+ Post New Thread
Results 1 to 7 of 7
  1. #1
    Newbie level 5
    Points: 212, Level: 2

    Join Date
    Mar 2019
    Posts
    10
    Helped
    0 / 0
    Points
    212
    Level
    2

    VU Meter only with 1LED (PWM) no IC except for OPAMP

    First of all yes it is homework but I worked on it and stuck at only this part. Maybe I am approaching on wrong side I am open to every advice.

    I need to design a environmental noise checker circuit. I am not allowed to use any IC except OPAMPS! Passive elements (resistors, capacitors, inductors, diodes), LEDs, Analog Microphone, DC Power Supply allowed.

    They are expected us to show sound level with only 1 LED by using time. To be specified; very low sound its off every time. above very low it should be open for 0.25 second and 0.75 off, above that 0.5 on 0.5 off and finally when its at highest it should be 0.75 on 0.25 off.

    So I managed to have a decent output from microphone by using non-inverting opamp. After that I obtained a square wave by using Schmitt trigger principle.

    Now I am thinking about how to use that time varying with respect to input amplitude. I know its somehow related with Pulse Width Modulation but have no idea how to construct something like that.

    Thanks

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 78,975, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,031
    Helped
    3631 / 3631
    Points
    78,975
    Level
    68

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    Hi,

    so you did some research...
    Really research about PWM? If so, you should already now the most PWM use a triangle shape waveform to compare with....

    Is this "hint" enough?
    --> go on this way.

    ***
    Show your ideas - even if not correct or not perfect.

    We help, but we wonīt give you a ready to use solution.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 81,957, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,443
    Helped
    4484 / 4484
    Points
    81,957
    Level
    69

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    Not PWM, what you need is an op-amp as a gain stage, a rectifier (either diode or better still an op-amp based precision rectifier) to give you a DC level proportional to the sound level. Then you need a monostable where the delay is set by that voltage. The monostable output feeds the LED.

    I think it should be possible to do it with a single quad op-amp IC but beware of the pitfall that you must reset the output completely when the voltage is zero or risk it staying on if it was already on and the sound level drops to zero.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 78,975, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,031
    Helped
    3631 / 3631
    Points
    78,975
    Level
    68

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    Hi,

    I agree with Brian.
    He thinks about an "analog" solution.

    But I followed your idea with PWM. This is the digital solution where the brightness is controlled via the duty cycle of the PWM.

    Both ways are possible (and a lot more). There is not the single one and only perfect solution.
    I think itīs the meaning of the homework to find "your own" solution.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  5. #5
    Super Moderator
    Points: 261,889, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,771
    Helped
    13912 / 13912
    Points
    261,889
    Level
    100

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    According to post #1, you want 4 discrete display levels (duty cycles 0, 0.25, 0.5, 0.75), no continuous variation of duty cycle. If so you need comparators (can be implemented with OPs) after the input amplifier and rectifier to distinguish the levels.



    •   AltAdvertisement

        
       

  6. #6
    Newbie level 5
    Points: 212, Level: 2

    Join Date
    Mar 2019
    Posts
    10
    Helped
    0 / 0
    Points
    212
    Level
    2

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    Yes I know how to implement comparators with opamps. But I didn't understand how they discrete levels of microphone output ? Aren't they work as giving output of their Vcc or -Vcc so doesn't it mean that I lose my microphone information, and still I couldn't understand how it seperates.



  7. #7
    Super Moderator
    Points: 81,957, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,443
    Helped
    4484 / 4484
    Points
    81,957
    Level
    69

    Re: VU Meter only with 1LED (PWM) no IC except for OPAMP

    You can't use the outputs of comparators directly because all you get is an on/off indication of the instantaneous level. You would see the output flash at the frequency of the audio instead of its amplitude.

    You have to convert the audio level to an analog voltage level (amplitude envelope) with an appropriate time constant before using the comparators.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



--[[ ]]--