help regarding 555 monostable-preventing false trigger

Discussion in 'General Electronics Chat' started by devalvyas, Jun 13, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I am using the following 555 monostable circuit for a 1 second pulse. The pulse drives an A.C. Universal motor for 1 second.

    Now i am faced with a problem where the circuit could be falsely triggered. I dont want the monostable to be re-triggered once its triggered for a selected time say 0.5 seconds. To be more specific...

    At time T=0, I give trigger pulse by opeaning the NC switch.
    I get out put pulse from 555 pin 3 for a duration of 1 second.
    At time T=1 sec the output pulse goes low.

    (i am ignoring the internal rise and fall time delays )

    Now i dont want to trigger 555, till time T=1.5 seconds even if i open the switch by mistake.

    This means that even after the output pulse of 555 is low at time T=1, the circuit shoud prevent the triggering till another 0.5 seconds.

    i.e. at time T=1.3 seconds if i again try to trigger the 555 timer by opeaning the NC switch, it should not be triggered.

    I should be able to re-trigger the 555 timer only after time T=1.5 seconds.

    How do i build this logic in this circuit?

    Can you please help?

    Thanks
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    555 won't retrigger during it's timing cycle. Your design has a few minor issues, but nothing too far out, and should work. R3 is too high, figure around 330Ω for a 14ma base current. Your timer is set for 10 seconds. T=1.1RC

    You will need a second monostable to extend the dead time past the monostable time, at least if I'm reading your post correctly. You might be able to add an RC time on your input conditioner to kill any button pressing for 1½ seconds.

    [​IMG]

    Have you seen these?

    Bill's Index

    555 Monostable
     
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  3. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    The R3 is only 120Ω not KΩ. The Rt is 900 Ω for a 1 second pulse...sorry my typo...

     
  4. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I have made the following schematics using two 555 timers. (file attached - see the second schematics)

    The NC switch opens and triggers timer T1. The output of T1 goes high and immediately triggers timer T2 as T2 is configured to trigger on positive going pulse. The output of T2 goes high immediately for time determined by 1.1 * RtCt = 0.99 sec.

    The output of T1 timer remains high for time determined by 1.1 * R4C6 = 1.54 sec. Hence even if there is a false triggering of switch NC during this 1.54 seconds it will not affect the output pulse of T2 which will time out after 0.99 seconds.


    Please let me know if this is ok and are the component selection right?

    but i am not able to figure out how to build the same logic without using two 555 timers? how do i add an RC time on the input conditioner to kill any button pressing for 1½ seconds?? please help me with schematics..
     
  5. flat5

    Active Member

    Nov 13, 2008
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    I think a two input nand gate could be used.
    Your pulse feeds one input.
    Output goes to 555 trigger.
    Other input has a pullup, an RC network and is connected to the gate output.
    Idea being the gate passes on the pulse to the 555 and turns itself off for awhile.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I started to reply yesterday, then realized it wouldn't work. Part of the problem is your unusual trigger input, the NC switch.

    Again, the schematics you show would almost work, though you tend to go towards extreme values. You don't need huge capacitances on pin 5, a 0.1 would work. 10ma drive to the transistors is all you need, not 40ma, and Rt can be much bigger so Ct can be much smaller (10µF is smaller than 1000µF). All of this affects current and size. The big thing about your second schematic is the first timer needs to set your ready to reset signal (1.5 seconds), and the second sets the actual on time for the motor (1 second). You have them reversed.

    Just curious, have you read my links?

    555 Monostable

    Bill's Index

    Anyhow, since I've basically decided I can't do what you want to do with the input conditioner, I'll show you how I would do it.

    [​IMG]

    Side note, .doc is not a good format for graphics. There are things you can't do with it on this site. Check this out.

    How to Display Attachments Full Size
     
  7. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I have read your link on 555 and found it to be very informative. As a matter of fact use it as a guide many a times.

    Regarding NC switch, its not an absolute requirement. I am actually using a micro switch as a trigger point. Like most of the micro switch, it comes with both NC and NO contacts. hence i can use any one of the triggers even a normally open contact. Is there a way to use input conditioner circuit for NO contacts to achieve the 1.5 sec time?

    Also in the first diagram R1, R2 and C2 is for input trigger signal conditioning. What does C1 = 1000 µF do?

    Also in second circuit where you have said "On both circuits" what it the role of C6 and C7?

    Please let me know.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    It sets the 1.5 sec delay, give or take, per request. It takes the 1000µF capacitor around 10 seconds to charge back up, where the switch will send a signal. The cap doesn't have to be fully charged to send the signal though, so the exact time is iffy. It may be a little long, but not too much so.

    A battery is a good source of juice, but capacitors across the power supply will handle unexpected transients, same way C3 and C5 do.

    One last point, the second schematic is much more precision on time control, more part for more predictability. It could easily be a 556. Check my albums out, look at the 555/556 datasheet for crossover info.

    http://forum.allaboutcircuits.com/album.php?albumid=41
     
  9. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Thanks a lot Bill.
     
  10. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    One little problem. I want the time on the first timer say T1 to be 'x' more than T2, ie. T1= T2+ x

    x= 0.5 seconds

    Now I am using a variable resistor Rt to control time for T2 from 0.5 seconds to 1.2 seconds. hence when T2= 0.5 seconds, T1= 1 sec and when T2= 1.2 seconds, T1= 1.7 seconds.

    Now I cant adjust two separate resistances, Rt and R4 to get exact time. Hence i have selected one common resistance Rt and another resistance R4 to just add 0.5 second time to what ever value of Rt i select....will this work... please let me know.....

    Attaching the schematics....sorry i am not able to attach other file hence attaching the doc file again.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    How about using the reset pin of the 555 to disable the 555 for a little while when it goes low? Something like this...

    [​IMG]

    You could even use a second 555 for precision control of the disabled period, and it would be set for how long the disable is, not total time. Since the 555 monostable trigger is the negitive edge this would also work with the chip.
     
  12. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Ok great. will try that out and post the reply. thanks
     
  13. Wendy

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    I think I may have made the RC time constant for R4 C4 a lot too long, but the principle applies.
     
  14. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    dear bill...apologize for the late reply...i tried the above schematic but its not giving me the trigger delay..i mean i can immediately trigger the 555 again once the output goes low....can you please advice...
     
  15. Wendy

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    Try measuring the voltage on pin 4. You should see a low lingering there after the timer times out. It won't be digital, maybe it has to be, but you should see under a volt for at least a second or 3. Are you?
     
  16. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I did the measurements. i connected a digital multimeter between pin 4 and ground.

    1. When I power on the circuit, the value on pin 4 does not immediately go to 12 volts. It takes around 1 minute to reach 11.70 volts.

    2. when i give the pulse to pin 2, the out put pin 3 goes high, closing the relay for 1 second as per the timer. i can measure about 7 volts on the multimeter before which it goes to zero again.

    3. When i give input trigger, relay closes, but the voltage on pin 4 jumps to 17 to 18 volts. and as soon as the output pulse goes low and relay opens the voltage on pin 4 drops to around 5 to 10 volts.and immediately starts to increase again. I am getting different reading on every connection.

    The voltage on pin 4 is not going down to under a volt as measured and definately not for 3 seconds.
     
  17. Wendy

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    Mar 24, 2008
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    2 and 3 are the same step, pin 2 pulse is the trigger. So where are you measuring 7 volts? You won't be able to see the pulse at pin 2, and pins 6 and 7 start at ground, slowly go up to 8V where it goes to ground at the end of the pulse. The normal voltage at 6 and 7 is ground, unless it is timing.

    Their needs to be a diode across R4, cathode facing Vcc, to keep Pin 4 from going way high.

    Are you using a protoboard? Try shorting pin 4 to ground, this should kill the circuit. I'll redraw the schematic to show the diode. The reason I want to try taking Pin 4 to ground is to see if your 555 still works.

    [​IMG]

    Here is how it should work.

    1. Power up, it will take Pin 4 over 10 seconds to reach 11V, maybe more. The 555 should not work for a portion of that.

    2. Trigger, The relay energizes for one minute, pin 4 may jump a little, but stays at 12V.

    3. Time out, the relay de-energizes, and it should take 10 seconds for pin 4 to reach 8 volts, give or take. It should not be possible to trigger the circuit for at least 2 seconds.
     
    Last edited: Jul 10, 2009
  18. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Thanks. I will try out the schematics and post the reply. I am using a breadboard for my circuit currently. The 555 is working fine as i am able to use the circuit properly with pin 4 connected to +vcc.

    When i power up the circuit, the voltage at pin 4 goes to 11 volts in about 50 seconds...its taking much longer than 10 seconds

    when i trigger the 555, the voltage at pin 3 goes to 7 volts before it goes low. it is not going all the way to 11 volts...its able to operate the relay fine though..

    Now, my timing cycles is much lower than 1 min and 10 seconds.

    Actually my output pulse has to stay high for around 0.75 seconds. The delay that I want in my re-trigger will be around 0.25 to 0.5 seconds max...i am trying to develop a zero speed or near zero speed sensor..i will post the entire idea and the schematics that i am thinking...in next post..

    In the designing part currently i am using output pulse duration as 1 second and the re-trigger delay could be as specified by you to test the circuit..once the concept starts working i will have to bring the re-trigger delay to not more than 0.25 to 0.5 seconds... will this configuration work for this fine timings or the orginal schematics you suggested using using two 555 timers be better ?
     
  19. Wendy

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    Mar 24, 2008
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    2 Timers would be better, much more precision overall. I'll wait until you're ready to try something new before putting pixel to paper.
     
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