Low pass filter to drive LED

Discussion in 'The Projects Forum' started by The_big_dill, Feb 24, 2013.

  1. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    Hi there,

    I am looking to make a sound sensor circuit that activates LEDs when the frequency drops below approximately 80Hz.

    My current set up is with a Sparkfun electret speaker with a built in amp connected to an Arduino, and it works great (however it is too sensitive)! Now i want to implement this with pure hardware.

    Have done some research and have many questions answered but i do need some clarification.

    1. I understand that the lowpass filter will be a simple RC circuit, to cutoff something like 80Hz with a 1microFarad capacitor i will need something like 1.6k ohm resistance; I used the frequency cutoff formula.

    Would i connect the input of this filter directly to an electret speaker (without amplification of input), or will that be ineffective?

    2. When cutting off the higher frequencies, i want to power something like a logic level transistor to drive the LEDs, any suggestions on how i would get the required voltage to power the transistor?

    Any suggestions welcome!
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    I think you mean an electret microphone correct?

    The mic should have some type of preamp/interface circuitry. The filter would likely be placed following that.

    Could you post details and circuit diagram of what you are using for more accurate help?
  3. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    I will draw a circuit for it soon, i am currently brain storming and unsure of whether or not i need to amplify the signal of the electret microphone. What does the microphone itself output? Would it be difficult to construct an amplifier circuit for it, what will i need?
    Last edited: Feb 24, 2013
  4. panic mode

    Senior Member

    Oct 10, 2011
    you definitely need to amplify it, mic output is in the mV range. also the RC filter will need different values for frequencies you have in mind.
    The_big_dill likes this.