555 one-shot latch

Discussion in 'General Electronics Chat' started by eyesee, Oct 19, 2013.

  1. eyesee

    Thread Starter Member

    Oct 19, 2013
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    I'm trying to design a monostable timer circuit that will not re-trigger at all. I know the HC221/HCT221 non-triggerable monostable ICs prevent a re-trigger during the monostable period but doesn't prevent a re-trigger after the monostable period has elapsed.

    I was wondering if I could somehow implement this function with a 555 alone or whether I'd need to also incorporate a latching circuit?

    Thanks.
     
    Last edited: Oct 19, 2013
  2. elec_mech

    Senior Member

    Nov 12, 2008
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    We need more information. What will trigger the circuit? You could use an edge-trigger, but this only prevents the monostable from re-triggering if the signal is held low. Example, if your circuit is triggered with a latching switch, the monostable will trigger when the switch is initially latched, but it will not re-trigger until the switch is un-latched and re-latched. In the case of a momentary switch, if someone presses it, the circuit will trigger but will not re-trigger until the switch is released and pressed again.

    If you want the circuit to trigger one time only, how will you reset the circuit?

    Another thought is to add a power-on trigger (see link above) so instead of using a switch, the 555 triggers when power is first applied and stays off until you cycle power to the circuit.
     
  3. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
    3
    The trigger input will be a photocell or a couple of bare wires to sense moisture. Can a 555 directly trigger on either of these?

    The circuit will be reset by switching it off and back on again.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A latching circuit should do it.
    What is the monostable pulse width and how accurate does it need to be?
     
  5. #12

    Expert

    Nov 30, 2010
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    Last week I told somebody to hold the reset pin (4) to ground to force an off and The RB told me that collapsing the voltage on pin 5 will have the same result. I'm thinking, SCR. Won't work on "reset" because it won't go as low as .7 volts, but it might wok on pin 5.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    AFAIK the 555 does not retrigger.

    As long as the starting trigger is gone by the timeout time you are fine, but THAT condition is not met by the type of sensors you have mentioned.
     
  7. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
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    The duration of the monostable needs to be around a minute or so. Doesn't need to be hugely accurate.

    The trigger may be continuous or occur again however, the monostable should just operate once and only reset when power is cycled.

    Can SCRs operate from 1.5V? If not, what type of latch would operate on such a low voltage?
     
  8. #12

    Expert

    Nov 30, 2010
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    If you can't buy one, you can make one.
     
  9. mbohuntr

    Active Member

    Apr 6, 2009
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    Darn it #12, you beat me to it!!! :D
     
    #12 likes this.
  10. #12

    Expert

    Nov 30, 2010
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    Some of us type with TWO fingers.:D
     
  11. eyesee

    Thread Starter Member

    Oct 19, 2013
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    I'm guessing most NAND ICs operate at around 5V to make an S-R latch?
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    CD4011 or CD4093 NANDs will operate at 3V - 15V.
     
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