555 Timer and Batteries

Discussion in 'The Projects Forum' started by svincent240, Dec 16, 2009.

  1. svincent240

    Thread Starter New Member

    Dec 16, 2009
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    I have a standard monostable 555 in part of circuit I am working on. I would like to power the 555 with 3 D batteries in series (4.5V) but whenever I hook up the battery power supply and trigger the timer the output stays high until the power is disconnected.

    The big problem here is that this does not happen when I use my wall adapter power supply (6V 2.5A). Also I have tested certain timers not to do this in isolated monostables on breadboards (NTE955M by NTE and NE555P by TI) but I am almost positive I had one timer do it when it was isolated that way. I think it may be the timer I used (an NE555N by ST) but I have no more to try outside of the large circuit. (CORRECTION: I just found another ST timer and isolated with the D batteries it did not stay on.)

    I should also add that this timers output is tied to Flip flop shift registers (sets one and clocks another).

    Its something that is kind of bothering me and I will be presenting the larger circuit as part of a senior project so I dont want to be caught off guard by a simple misunderstanding.

    Thanks for the input.
     
    Last edited: Dec 16, 2009
  2. Wendy

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    Mar 24, 2008
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  3. SgtWookie

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    Jul 17, 2007
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    You are right at the minimum operating voltage for a bjt (transistorized) 555. If you're below 4.5v at all, operation will be very "iffy".

    It would help a great deal if you post a schematic of your circuit.

    [eta]
    You might consider changing to a CMOS 555. They have a much lower minimum operating voltage. Their big limitation is how little current their output (pin 3) can source; about 10mA when Vcc=15v, far less (around 1mA) at Vcc=4.5v.

    You could always use a voltage follower or PNP transistor to source more current if need be.
     
    Last edited: Dec 16, 2009
  4. svincent240

    Thread Starter New Member

    Dec 16, 2009
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    Thanks for the responses.

    Yes, the monostable ckt. I'm using is the same as the one you linked to.

    As for the 4.5V problem, I have the same issue when using 2 9V batteries in parallel (for extra current).

    When using the 9V source the timer heats up while the output is high. It seems that the flip flops on the output are pulling current from the 555. Again the 9V from my wall adapter causes neither problem.

    I might have a decent schematic soon that I will post if I can. I am doing this as part of a senior project and because of time constraints I am planning on just using a lab power supply. The reason I am asking is just for future reference and as something to share in my presentation.
     
  5. SgtWookie

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    Jul 17, 2007
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    Well, even though your circuit may be very similar to Bill's circuit, we really need to see exactly how yours is wired up.

    Bill's circuit doesn't have any flip-flops.
     
  6. svincent240

    Thread Starter New Member

    Dec 16, 2009
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    I will try to post a schematic as soon as I can
     
  7. SgtWookie

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    Jul 17, 2007
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    OK.

    Keep in mind that if you are using 54/74 series TTL flip-flops/shift registers, they need to be supplied with 4.5v to 5.5v; they weren't designed to be operated from 9v.

    4000 series CMOS IC's can be operated from 3v to 16v or so.
     
  8. svincent240

    Thread Starter New Member

    Dec 16, 2009
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    The flip flops in the shift registers are all Motorola mc14013BCP D flip flops.
     
  9. SgtWookie

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    OK. Motorola uses an MC1 prefix to their 4000 series CMOS IC's.
    Phillips uses a HEF prefix.
    Many other manufacturers use a CD prefix.
     
  10. Wendy

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    Mar 24, 2008
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    Note, you have to have a signal conditioner like my design has, or the 555 will stay high. This was explained in the associated article.
     
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