555 timer - odd behavior

Discussion in 'The Projects Forum' started by Otaku, Jun 2, 2009.

  1. Otaku

    Thread Starter Active Member

    Nov 19, 2008
    128
    2
    Hi all,
    I'm having trouble with this circuit and need some insights. It's configured as an oscillator, but does not behave correctly under certain conditions. Power is applied to pin 8 (+12VDC) at all times. The circuit is activated by switch closure (actually a Darlington) which completes a ground loop to pin 1.
    When a NE555P chip is installed in this circuit, it behaves normally. When pin 1 is grounded, the 555 immediately goes high (~10VDC) for ~0.5 secs and goes low for the time set per the 100K trimpot. This cycle repeats until the ground connection to pin 1 is removed.
    When a MC1455 chip is installed and power applied, the pin 3 output immediately goes high (~10VDC) without pin 1 being grounded. When pin 1 is grounded, the 555 goes low for the time set per the 100K trimpot, and does not oscillate. When the time runs out, the pin 3 output again goes high.
    The diodes in this circuit are both 1N4148's.
    Does anyone have an idea as to why this is happening? All advice is appreciated. Thanks!
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit is missing the diodes that prevent the very high flyback voltage produced by the inductance of the relay's coil from messing up or destroying the 555.
     
  3. Otaku

    Thread Starter Active Member

    Nov 19, 2008
    128
    2
    Would a 1N4148 across the relay coil do the job? And which direction would the diode ring be facing - toward pin 3 or to ground? Thanks much!
     
  4. Otaku

    Thread Starter Active Member

    Nov 19, 2008
    128
    2
    Audioguru,
    You mentioned in another post of mine (How much current is needed?) that pin 1 of the 555 should not be floating and that this prevents the 555 from going low. Pin 1 is only disconnected until the circuit is triggered via a Darlington which completes a ground connection to pin 1. This configuration had been working without a problem until a MC1455 timer was installed. The NE555, 555CN and LM555 chips have all worked fine. When the MC1455 is installed, pin 3 goes high when the circuit is powered up - this never happened previously. It's very puzzling. Thanks again for your feedback.
     
    Last edited: Jun 3, 2009
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Pin one is one of the power supply leads, the minus lead. Pin 8 is the + lead, and pin 4 is enable, so it should be high.

    Have you gone thorugh this?

    The 555 Projects
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Pin1 of a 555 should always be connected to 0V.
    Your circuit works oddly and you found out that the MC1455 works differently than other 555s in your circuit so don't use it.
     
  7. Otaku

    Thread Starter Active Member

    Nov 19, 2008
    128
    2
    I think that's the best course right now. I'll check with the supply houses to see what's available.

    Bill - thanks for the link to the 555 projects, lots of good info there.
     
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