555 for Bill Marsden

Discussion in 'The Projects Forum' started by KMoffett, Nov 22, 2013.

  1. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    I know you're the 555 aficionado around here, so I thought I would see if you have done this. Am I reinventing the wheel? A poster on another forum was trying to gate a 555 astable, but was unhappy about the results of his experimenting. He had tried to remove the power from pin 8 (Vcc), and also tried to ground the pin 4 (Reset). His problem was that the first cycle after ungating was always longer than the normal running cycles. Of course this was because the timing capacitor was fully discharged with both of these options, because the capacitor has to charge from 0V to 0.66*Vcc. After that it would cycle between 0.33*Vcc and 0.66*Vcc. I thought if there was a way to start the capacitor at 0.33*Vcc when gated-on, that would solve the problem. I came up with a way that works. (attached) I clamp the capacitor to a voltage divider through a N-MOSFET. The MOSFET then gates the 555 by allowing the capacitor charge or not.

    Ken
     
  2. #12

    Expert

    Nov 30, 2010
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    It looks valid, assuming SW is a gating device like a bipolar transistor, an open collector comparator, or several other devices.
     
  3. Alec_t

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    Sep 17, 2013
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    A voltage-divider and diode can also do the job.
     
  4. Alec_t

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    Here's another way, using a resistor between pins 5 and 7 to reduce the lower switching threshold.
     
  5. GopherT

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    Nov 23, 2012
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    I haven't tried, but can pin 7 be directly connected to pin 5 without a resistor to make the lower threshold zero volts?
     
  6. #12

    Expert

    Nov 30, 2010
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    If you do that, you're asking the internal comparator to operate with zero volts on its inputs. Some comparators are built to do that. I guess it's a check and see proposition.

    edit. I should have looked up the part instead of assuming I knew what you were describing.
     
    Last edited: Nov 23, 2013
  7. Alec_t

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    LTspice doesn't think so. A minimum value of resistance, which is dependent on the cap charge/discharge resistors, is necessary.
     
  8. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    Yeah! I like that too.
    Thanks Alec.

    Ken
     
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