555 based fading LED ckt mod

Discussion in 'General Electronics Chat' started by MadJ, Feb 25, 2011.

  1. MadJ

    Thread Starter New Member

    Feb 18, 2011
    11
    0
    I am using the following circuit to create a pulsing/throbbing LED effect.
    [​IMG]
    The LED is being driven by the saw tooth generated at the capacitor.

    It is working fine, but the problem is, the maximum brightness of the LED is very low. Inspite of connecting a transistor, the LED won't glow to its full brightness. I tried various modifications but nothing seems to be working.
    Can anyone please suggest me a suitable mod so as the LED lights up to its full brightness when the i/p signal is at its peak.

    I referred this : http://www.youtube.com/watch?v=tQQp8_WPcmo

    EDIT : Just noticed, this is the same circuit
    http://forum.allaboutcircuits.com/showpost.php?p=117649&postcount=13
    check Figure 2.1
     
    Last edited: Feb 25, 2011
  2. MadJ

    Thread Starter New Member

    Feb 18, 2011
    11
    0
    Increasing the Vcc from 5v to 9v solves the problem.
    But is there any other way to keep the Vcc at 5v and still get a bright light ?
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Yes, but it is more complicated. My article includes your circuit, and then covers the other alternatives.

    LEDs, 555s, Flashers, and Light Chasers, Chapter 12, Special Effects

    *************

    Side note, it doesn't have to be a 555. You can use two 555s and a LM393, or a sincle LM339 with the concept. You can wire up 2 hysteretic oscillators (one slow, one fast), use a 3rd comparitor to PWM it, and not use the 4th comparitor. A simple 555 gets rid of a lot of resistors, but then you have the chip.
     
    Last edited: Feb 25, 2011
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The timing capacitor charges to 0.67 times the 5V supply. (3.35V)
    The red LED is probably 2.0V.
    The base-emitter voltage of the BC547 transistor is about 0.68V so the 120 ohm resistor has a voltage across it of (3.35V - 2V - 0.68V)= 0.67V and creates a current in the LED of only 0.67V/120 ohms= 5.6mA so the LED is dim.

    If you replace the 120 ohm resistor with a 27 ohm resistor then the LED will become much brighter with 25mA.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    But the LED will go off when the triangle voltage drops below about 2.5V, and the triangle will go all the way down to 1.67V.
     
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