555 Timer problem driving 2 LED's

Discussion in 'The Projects Forum' started by Mint Electronics, Jul 7, 2012.

  1. Mint Electronics

    Thread Starter New Member

    Jul 7, 2012
    I have constructed the following circuit:
    And it is not working the way I would like it to work. L1 as seen in the schematic is not lighting up as equally as my L2 is. Is the the standard behaviour of the 555 timer?
    I am driving the circuit using 12V and a 1k resistor with each LED, the current at L2 is 8.4mA and at L1 it is 7.6mA.:confused:
    Can somebody please help me out?
    - Thanks in advance
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Must be the ic not sinking as well as sourcing,i wud lower the resistor value in the led thats dimmer, to balance the lights.
  3. Mint Electronics

    Thread Starter New Member

    Jul 7, 2012
    Yep I did that and it works perfectly now. However just before I solder this on, should I maybe get another 555 IC or are they all like this?
  4. Pencil

    Active Member

    Dec 8, 2009
    Very likely they will all be like this to some extent.

    The reason for this can be seen in the datasheet.
    Note the Voh (output high voltage) versus the Vol (output low voltage).

    The Voh will be about Vcc-2, where as the Vol will be only
    about .1 above ground. Simply put, the voltage across R4 and L1
    is probably in the neighborhood of 10V (12v-2v), but the voltage
    across L2 and R3 will be near 11.9 (12-.1). This discrepency would
    result in the need to calculate different resistor values to achieve
    the same current through each LED.

    Check the datasheet for exact specifications of your IC.
  5. Wendy


    Mar 24, 2008
    555 Schmitt Trigger

    555 Hysteretic Oscillator

    If you look at the internal construction of the 555 shown in the first link you will note the Vcc side of the output uses a Darlington pair, while the ground side is a simple transistor. This means the Vcc side gets with 1.3V of Vcc, while the ground side gets within 0.1V of ground. If you want equal intensity you need to adjust the voltage differences.

    Different LED colors also drop different amount of voltage. This has to be compensated for too.

    LEDs, 555s, Flashers, and Light Chasers