LED (Sound Reactive) Circuit

Discussion in 'The Projects Forum' started by dino_, Feb 25, 2012.

  1. dino_

    Thread Starter New Member

    Feb 25, 2012
    I am using 9V, 2 resistors, one is 1kΩ - another one is 5kΩ.

    Also NPN transistor which is the key of this circuit, but.


    This is schematic. I am using 2N2222A transistor, I'm sure this schematic is correct, but transistor may not be, any thoughts?
  2. dino_

    Thread Starter New Member

    Feb 25, 2012

    Any help on this?
  3. Audioguru

    New Member

    Dec 20, 2007
    If the LED resistor is 1k then a 1.8V red LED will have a current of about 7.1mA which is not bright.

    For a saturated collector current of 7.1mA the base current should be 0.71mA.

    When the base resistor is 5k then its input must be 36V to produce a base current of 0.71mA. A peak voltage of 36V (81W RMS into 8 ohms) from an audio power amplifier will destroy the base-emitter diode of the transistor since its maximum allowed reverse voltage is only 5V.

    If driven from an ipod headphones jack then the peak voltage is only 2V full blast and the base resistor should be 100 ohms to 180 ohms.

    The audio signal is AC but the transistor and LED operate from DC. Then the AC signal lights the LED only half the time which causes it to be dimmed with an avberage current of only 3.5mA.