Disco LED Circuit Newbie

Discussion in 'The Projects Forum' started by Tvbox, Jul 19, 2010.

  1. Tvbox

    Thread Starter New Member

    Jul 19, 2010
    Hello, this is my first circuit project. Bought a few books to learn about components and basic circuits, i also bought an op amp book with circuits needed to further my knowledge.

    Basically i wanted to build a giant circuit that would pick up bass through a mic then would light up LED when the bass hit (can put it in a car, computer case, Frame, etc..). I will be postin updates and would appreciate it if any one who spotted obvious mistakes or opinions on what would or would not work.

    The project in theory would have a few stages:
    1.Mic Preamp (powers and pics up sound waves for mic)
    2. Low Pass Frequency filter (filter out frequency > ~100hz)
    3. Audio Power Amplifier (bring up amp and volt input to usable output??)
    4.Peak Detector (detects each bass hit)
    5. Comparator circuit that would initialize the lights to turn on if input is > than ground.
    6. Finally, IC to control LED lights.

    Ive started and believe i have accomplished step 1, this is my current circuit:
    1x NTE975 IC General Purpose Op Amp (http://www.nteinc.com/specs/900to999/pdf/nte975.pdf)
    5x 56K resistors
    3x 10μF Electrolytic capacitors
    1x 8Ω Dynamic Mic from radioshack (http://www.radioshack.com/product/index.jsp?productId=2062216)
    1x 9v Battery
    The Schematic looks like this except R1 is 28K and R2 is 56K:[​IMG]
    I built the circuit but theres no way i know is its doing its job, the output at pin 6 is 4v droping down to .1v then poping back up to 4v again the amp is at .2mA from the battery and at the output from the IC its at .01mA -.02mA, that seem right to any one? any one know how i could test this or the next step, low pass filter would involve with these measured ratings?
    Help and advice is greatly appreciated!!
    Thank You.
  2. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Your circuit is oscillating at a low frequency because its voltage divider biasing with the two 47k resistors need additional filtering because the power supply voltage is jumping up and down due to the opamp's current fluctuations.

    The electret mic (it is not 8 ohms, it is about 3k ohms) will not work unless it is powered from a filtered voltage.
    You don't know the voltage at the mic so the input coupling capacitor should be an unpolarized film capacitor. Then the voltage divider should use 100k resistors.

    An extremely simple RC lowpass filter is made with a capacitor in parallel with R2.

    Try this:
  3. Tvbox

    Thread Starter New Member

    Jul 19, 2010
    oh okay, thank you for your reply, so i actually just tested the resistance of the electret mic and it read 1.5k ohms. I know the mic has 3 cables im just connecting ground and positive, there 2 positive cables and i connected the thicker one, that okay?
    I will try out the new set up thank you, few more questions:
    i deleted the 100uf capacitor from + 9v battery to chassis (since i dnt have one) that effect anything? and the new schematic has the 47uf capacitor to chassis, i should run it the the ground connection? since i dont have chassis?