555 timer 1.5seconds delay.

Discussion in 'The Projects Forum' started by kwanchong, Dec 21, 2010.

  1. kwanchong

    Thread Starter New Member

    Dec 20, 2010
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    How does an AND gate work if inputs 111 produces an output '1' if the TRIGGER is active-low? :confused:
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    OK, my bad. A NAND gate then.

    I suspect anyone could figure it out once the concept is in place.
     
  3. kwanchong

    Thread Starter New Member

    Dec 20, 2010
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    Thanks. I've connected the timer and i don't seem to have any time delay. please do take a look at the attached image.
     
  4. kwanchong

    Thread Starter New Member

    Dec 20, 2010
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    Hi, I'm currently working on a small car that moves forward until a counter counts till 7, then stop for 1.5 seconds using a 555timer in monostable state, before reversing again. The counter is fully operational but I'm having problems with my 555 timer.

    As shown in the image below, the 3inputs of the NAND gate is coming from the LSB to MSB of the mod-8 counter that I'm using to count to 7.

    Please do offer me suggestions on how to solve this problem I'm facing.

    Thanks!
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    This should have been with your old thread, Need some help on 555 timer..., since it is a continuation of the same problem.

    I apologize, I thought you knew more about 555s. This is found in the 555 datasheet, datasheets are always your friend. A proper 555 monostable must have a signal conditioner. Complete schematics and theory of operation is found here...

    555 Monostable

    You need to add R3 and C1 shown in that schematic.

    BTW, this article has been released to the AAC book as 555 MONOSTABLE MULTIVIBRATOR.

    This is another circuit I drew showing the same schematic (different parts designations).

    [​IMG]
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Since you have opened a new thread on the subject (not a good idea since it will confuse the issue dramatically) I will continue the discussion there.

    555 timer 1.5seconds delay.
     
  7. kwanchong

    Thread Starter New Member

    Dec 20, 2010
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    Hi, sorry for the confusion, i was kinda desperate for answers because the deadline is coming up and we still need to order the components and to solder the components. I've done exactly what you've drawn up. however, is it possible to stop the counter at 7 and not continuing to count from 0-7 repeatedly?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    @ Bill_Marsden,
    Thanks for reporting the extra thread.

    @kwanchong,
    I merged the replies on the other thread to the new opened thread.
    This may cause some stange view, but all posts are together now.


    Bertus
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Thanks Bertus.

    kwanchong,

    You almost have everything you need. Looking at it you will need to invert the 555 output and feed the signal to a AND gate to block the incoming signal to the counters. This was why I said this resembles a freq counter, it is the same way it works. You could use a 4023, it is qty 3 3 input NAND gates, two of the gates can make a two input AND gate. You will still need an inverter, you can use a 4049.

    Or you could use a 4049 hex inverter and a 4081 quad AND gate to make the logic.

    Important! Do not forget bypass capacitors for each chip, and more importantly, do not forget to tie unused inputs of unused gates to ground!

    Could you post your counter schematic again? I think it got lost in the shuffle.

    I would draw the schematic for you, but this is obviously homework, and we try to help short of doing the work. This is very much a teaching site.
     
  10. kwanchong

    Thread Starter New Member

    Dec 20, 2010
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    Hi Bill_Marsden, the counter is attached below. There seems to be a slight problem with the 555timer as well, the output stays high forever after the counter counts to 7, it does not go low. I'll attach the timer and the whole circuit as well.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    The 555 circuit will work if wired correctly. Ergo, you have a wiring error somewhere. The 555 article explains why this is so (see theory of operation). There is no circumstance you would have 2.5V on pin 3, the 555 will either be a high (3.8VDC) or a low (0.1VDC), with a 5VDC power supply.

    There needs to be a 2 input AND gate between the signal source and the counter. The 555 feeds the second input of the AND gate.

    Are you doing this only with a simulator?
     
    Last edited: Dec 23, 2010
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