Connect with us

555 timer

Discussion in 'General Electronics Discussion' started by mav2, Apr 14, 2010.

  1. mav2

    mav2

    5
    0
    Apr 14, 2010
    hi all i am trying to build a auto cancell circut for m bike, i thought of using a 555 timer, i need t to stay on for approx 20 seconds then click off and not come back on till it is triggered again, i have a circut built, problem is it will not go off until i use the reset, is there a way to have it auto reset after 20 secdonds or so?the output will be driving a small relay which will be supplying power to my flasher in the bike..anyone with any ideas??
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    What, is it that you need it to stay turned on for 20 seconds after power is applied and then turn off?
     
  3. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    When you start dealing with time-frames like 20 seconds, you really ought to consider a micro. Something like a PIC10 or PIC12 series is available in the same package as a 555 and can do all sorts of fancy things that a 555 can never do. And the best part, no external components other than a bypass cap!

    ---55p
     
  4. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    R2 comes out to be 290 Meg. How exactly do you get that out of a 1Meg pot in series with a 100K pot?

    Hint: You can not do 20 seconds of anything from a 555 timer.

    At these durations you need to count down from a faster clock or use a micro.

    ---55p
     
  5. mav2

    mav2

    5
    0
    Apr 14, 2010
    ok, i hear what you say but i guess the big question is what is the bestway to do this any ideas or schematic drawings ? isthere a place i canget moreinfo?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    No, the first thing is that you need to describe exactly what you want it to do. We're still unsure. 55p points out that 20 seconds is too long for a 555.
     
  7. roltex_rohit123

    roltex_rohit123

    92
    0
    Oct 12, 2009
    I'm really sorry i took it for an astable one. i havent tried monostable yet. but from what i calculated from 555timer pro it gave me the following
    R=2M
    C=10uf
    pulse time is 21.97sec

    http://i820.photobucket.com/albums/zz124/rohitdhamal/monostable.png


    so the best option remains is use a 1.5Mresistor with a 500k pot. you will get it. i think i'm not wrong this time.
    really sorry for the misguidance in the previous post.
     
    Last edited: Apr 15, 2010
  8. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    As 55p has pointed out, the 20 seconds is just too long for a 555... 2 megs of resistance, charging a 10uF cap? That wouldn't be close to stable (even possible?) unless you got a cap with nigh ZERO leakage. You're talking 6uA of current charging your (laaaarge) cap.
    That's not practical at all.

    Note the little warning in the lower right part of that wizard about pin 6? Heed it! :)
     
  9. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Actually, it would be under 1 micro-amp, not 6. What you have to worry about is just before the cap reaches the high side comparator threshold of 2/3 of the input voltage. Assuming you are running at 5V, the threshold is at 3.33V, the resistor has about 1.6V across it and is delivering under 1 micro-amp. Not possible with any technology that exists today.

    ---55p
     
  10. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Oh yes you are! At least you are consistent. You are consistently wrong!

    ---55p
     
  11. mav2

    mav2

    5
    0
    Apr 14, 2010
    555 timer.

    hey again all, i think i have a solution to my problem, testedit and it seems to work, a littlebigger then i wanted, but i was in the garage and found a omron h3ca-a relay, set it to mode d and dialed in 20 seconds, and it works perfectly, wheni trigger it it will turn on the built in relay for 20 seconds then shut it off, and if i want it longer i can do that, i am assuming they are using a micro chip to do this, didn'ttake it apart yet...lol anyways thanks to eeryone for their input....
     
  12. roltex_rohit123

    roltex_rohit123

    92
    0
    Oct 12, 2009
    there was a warning abt current not met by pin. there r tolerances which account for changed timings. but still it should work like it. and if it is in the off position then reverting the polarity of connection ie positive instead of ground. i just made one timer of 4min 45 sec off and 2 min 10 sec on astable from the 555 timer pro. the 555 timer pro should work fine this time also. well if its simulation data that proves wrong then go for actual practical. i was nt able to do it because i'm going for my PhD.entrance this sunday. so i dont have much time from studies. but i'l definitely look over this. this time i made the timing calculation with 47uf capacitor and 365K resistor. now there is no warning also. please see if it works...
     
    Last edited: Apr 16, 2010
  13. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    The little screen cap he posted said 12V. Either way, we're in agreement... no way will it work. :)
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    I note that the original poster seems to have found an alternate suggestion. I think it would be a really good idea though if we started a thread on long duration timers with the aim of coming up with a resource for people who want this sort of thing (because this comes up time and time again).
     
  15. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    You are right. I missed the 12V part.

    But, the point I wanted to make got lost in other things. Your analysis of 6uA is valid when the cap is discharged. But that is not the state we need to worry about. The worst case happens when the cap is charged and we are just below the upper trigger threshold. At that point, the voltage across the resistor is 1/3 of the supply voltage and the voltage across the cap is 2/3 of the supply voltage. It is at that stage that the cap has the highest leakage current and the resistor is providing the least current, about 2uA in this case with 12V supply.

    Chances are that the cap will never get this charged. The resistor and the leakage currents will reach equilibrium at some voltage less than the 2/3 point and the system will just sit there forever with the cap charged part way and nothing happening.

    ---55p
     
  16. tronixstuff

    tronixstuff

    9
    0
    Mar 23, 2010
    personally, I would use a PICAXE -08M microcontroller, switching a relay on and off.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-