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How to auto trigger a 555 timer

Discussion in 'General Electronics Discussion' started by abuhafss, Aug 31, 2010.

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  1. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    I attach a flow chart of my project.

    JOB 1 and JOB 2 are identical processes.
    All timers are proposed to be 555/monostable.
    JOB 2 will be completed 10 sec after JOB 1.
    And then the cycle will be repeated, thus there will be an interval of 10 sec between each job.

    Questions:
    1) How one timer (Timer A) will trigger the next timer (Timer B)? I mean how will I connect Timer A with Timer B?

    2) Studying the flow chart, can somebody guide me if I am correct. And do I need any improvement in the usage of the timers?
     

    Attached Files:

  2. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    can you supply a circuit diagram or give more information?

    cheers
     
  3. NickS

    NickS

    367
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    Apr 6, 2010
    I think we discussed this same thing in this thread
    https://www.electronicspoint.com/triggering-555-another-t224804.html
    Basically you use the reset pin. And if you need alternate logic use inverters.

    This forum has also discussed to exhaustion the difficulties involved with using 555's for long time delays. see here

    However you can use this device ICM7242IPA+ which is a timer and counter so you can set the timing in a healthy range and then divide down to the seconds of delay you need.

    Hope that helps
    -Nick-
     
  4. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Here is the final schematic what I have designed after consulting some senior members.

    Please help me and check if it will work as per my proposed time graph.

    Thanks for the help.
     

    Attached Files:

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    The senior members you discussed this with don't seem to understand that a 15 second period generated with a 555 is not likely to be very stable.

    Personally, I'd go with a single 8 pin PIC (a picaxe would be even easier), and a couple of transistors to drive the relays.

    An alternate option is to use an oscillator and a divider to get a stable 5 second period oscillator, then drive a 4017 set to reset on the 7th count. Use OR gates (or wire OR with diodes) to get the series of outputs you want (the whole thing repeats every 30 seconds (6 time periods)

    [​IMG]
     

    Attached Files:

  6. NickS

    NickS

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    Apr 6, 2010
    Good advice Steve, sounds familiar but perhaps he will actually listen to you.

    At this point I think he should try breadbording it to see if it works? Theres no teacher quite like doing it.
     
    Last edited: Sep 3, 2010
  7. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    I have very limited experience of electronics, especially I have never played with PIC or PICAXE. I shall be grateful, if you can help me in making my 555-based circuit really workable.

    Thanks.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    Abuhafss, are my posts invisible?

    Testing, testing...

    Unless you plan on using a series of 555s as an academic exercise (and even then as a very poor one) I have suggested a much better solution using a 4017.

    You can even (at a push) attempt to use a 555 as the oscillator (just never quote me on that).

    If you can understand your 555 circuit, (I gave up) then what I proposed should be as clear as day.

    OK, feel free to ignore me. I really don't care. But several people have told you that using 555s like this is a recipe for poor results. Yet you still ask for advice on making it work.
     
  9. NickS

    NickS

    367
    0
    Apr 6, 2010
    Why don't you ask your "senior members". I think they are the only people that want anything to do with your design in the form you demand it to take. The IC I recommended requires no uC. it is also just as simple to use as a 555 so don't tell us your experience prevents you from considering our advice.

    If you care to take our advice in this forum then perhaps we would care to help. But when you ignore our advice(that you asked for) what makes you think we care to waste any further time helping someone who is not going to listen anyway.
     
  10. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    With due respect, both of you are getting me wrong.

    If I wanted to ignore your advises why should have I replied you. I'd love to use either of the two devices if I knew about them. You might not believe me, the very 1st thing I did after receiving your suggestions was that I searched the net to learn about the features, pin and tried to find some sample circuits. I learned a bit about the pins, and some rough idea about their features. But couldn't find sample circuits for better understanding.

    So I postponed my learning, for some other time when I could find the sample circuits. In the meantime, I requested you for my 555 circuit. I apologize, if that has hurt you. That's what I can say.:eek:
     
  11. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Okay guys, found a few basic circuits for 4017. Do we always need a 555 timer to feed CLOCK of a 4017?

    For ICM7242, I couldn't find yet. Shall appreciate your assistance.

    By the way, I am considering to make a change in my circuit. Now, I will have only 2 timer set up in series ---- 5 sec and then 15 sec. (time tolerance 2% acceptable). Do you still suggest that 4017 or ICM7242 would be a better option?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    Please go back and read through this thread. The answers are there.
     
  13. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    For a 22uF capacitor, and where the time required is 15 seconds, the series resistance is about 620k.

    The trigger point for Vcc of 15V is about 10V, at which time there will be 5V across that resistor, and so the current through it will be about 8uA. According to the 555 specs, you may see 0.35uA leakage through pins 6 and 7 and about 3uA through a typical electrolytic capacitor.

    This means that almost half the current used to charge the capacitor is leaking away. Leakage tends to be quite temperature dependant, so this figure will not be constant. If you see temperature variations, you will see timing fluctuations in excess of what you might expect.

    You are in the region where the normal calculations for resistor/capacitor values for the 555 do not apply, but probably not yet close to where the device simply fails to ever get to discharge. The timings are not easily predictable, but are probably reasonably repeatable.

    Practically, because you're almost certainly going to set these resistors by observation (you've considered this, right?) -- the initial approx 2.5% inaccuracy of the 555 (which incidentally is only for R <= 100k) is probably OK.

    Again, from memory, I think you specified 2% accuracy (on checking, I note that you say 5% is acceptable).

    I would not be comfortable with expecting 2%, but 5% is achievable. It does depend somewhat on the capacitor -- I would be looking at using a tantalum device.

    However a major issue is that the timer will spend a lot of its time with the capacitor voltage near the trigger point. During this time it will be sensitive to noise. I would recommend that you ensure that your power supply is quiet and that the 555s' power supply pins are bypassed.

    What is the impact of one of the 15 second monostables triggering early?

    Would I use 555's for this? No.

    Would I advise you to use them? No.

    It looks like you've latched on to a particular idea and want to make it work when there may be better and simpler solutions that do not suffer from the drawbacks that this method does.

    This particular method also requires a button press to start it. Is that a requirement? There is no way to stop it (other than powering off) -- is that what you want? Maybe those are un-stated requirements and possibly tend to favour one approach over another.

    One of your responses in another forum suggested how simple a picaxe program would be to solve this (and personally I think it's probably the simplest option). Had you come back to us with the question of how to drive a relay from a picaxe, we could have answered very quickly (it's one resistor, a transistor, and a diode).

    I note that we are your "friends" in other forums, and they are your "experts" here. Why do you feel the need to keep your other sources for assistance anonymous? Nobody here is going to get upset if you say "the people at xxxxx say...".

    You got some very good advice from allaboutcircuits.com, such as the need to have a resistor in series with the pots (I assume trimpots), and issues concerning reset at power on. Incidentally the issue of reset at power on is something that could cause you lots of grief if 2 or more of the monostables decide that they're triggered at power-on. You could end up with 2 signals chasing each other around the loop, rather than the single one you want.
     
  15. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Hi Steve

    I really appreciate your effort, sparing your time to convince an un-experienced like me with a detailed explanation. I understood your explanation.

    A good news; I have decided to complete my project with PICAXE because it appears to be more easier. I have already started learning about it. Frankly, for me it was bit scary how to handle those many pins. But, today you have encourage me a lot. I hope I will soon be able to make my first program. I had a very good programming experience of BASIC language, hope that will help me a lot.

    By the way, you being experienced and knowing my status, what do you suggest ...... from where should I begin i.e. a PIC or a PICAXE? Cost is not a big problem.

    Regarding the other forums, I have already stated that I had misunderstood that talking about other forums is banned.
     
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