Audio Peak Detector Circuit - Will this work?

Discussion in 'The Projects Forum' started by bluesguitar03, Apr 3, 2009.

  1. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    I've tried to do my research as best as I can and I think I've worked up a pretty good circuit. I need to light up an LED when the level of signal (audio from a mic preamp) goes above a certain level. The actual voltage for threshold is not determined yet which is why I have the pots R1 & R5 in the circuit. I had them as fixed resistors that set the reference voltages at 7.77V and 4.23V (for +1.77V and -1.77V peaks) but will end up using trimmer pots to allow for fine tuning.

    My 2 concerns are if the LED will light up long enough to be noticeable on quick peaks over the limit, and I want to make sure that adding this circuit after the preamp will not cause any detrimental effects on the audio.

    I appreciate any and all comments... thanks for your time!
  2. flat5

    Active Member

    Nov 13, 2008
    To make sure you notice the peak, use a comparator and detector to charge a capacitor to hold the LED on for a second or so.

    To keep the audio clean branch off from the audio chain without loading the circuit and do this other process off the main track, so to speak.
  3. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    Could I get away with just putting a capacitor across the LED and it's resistor?

    I will increase the resistor at the input to reduce loading.
  4. DickCappels


    Aug 21, 2008
    Just a thought: If you want some real control over the minimum time the LED is on, you might want to consider driving the LED from the output of a retriggerable monostable, like the good old NE555 (or the more modern LMC555).

    From your description, wanting the LED to light when the signal exceeds some level, leaves me wondering why you are using two comparators.

    I haven't tried this, but it might work: If you eliminate the lower comparator (U1B) and transistor (Q2), a small capacitor from the collector of Q1 to pin 4 (the inverting input of U1A) will turn your circuit into a one-shot. The value of the capacitor will determine a minimum on-time for the LED.
  5. thatoneguy


    Feb 19, 2009
    Not unless it is a Big Capacitor (close to millifarad).

    See post above for better method (cap on base drive of transistor)
  6. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    I am using two comparators because I need to have it signal if it exceeds a limit in the positive direction or if it exceeds a limit in the negative direction.

    The capacitor from the collector of the transistors to pins 4 and 7 (both of the reference points) seems to have done the trick at least as far as the simulation is concerned.....

    Thank you very much!

    If you have any more ideas on how to improve this design, let me know!
  7. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    Here's the circuit I have now. The only real difference in this one compared to the previous one is the capacitors C3 and C4 that were described above. In a few weeks I will hopefully get around to testing it in its application. (Vacation starts Monday!)

    Will probably mess around with the input resistor to see if there is an improvement with using a larger value.

    Thanks again for your help!