simple time delay switch- 555 or simple rc network with transistor

Discussion in 'General Electronics Chat' started by devalvyas, Jun 19, 2009.

  1. devalvyas

    devalvyas Thread Starter Active Member

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    I am trying to build a simple time delay circuit. I have used a 555 monostable configuration for that. schematics is attached.

    Now I want that the timer should start when ever i give a trigger input.

    now 555 times out irrespective of whether there is an input pulse during the timing cycle.

    To give more clarity, the attached schematics is of 555 configuration with time of 10 seconds output pulse.

    1. At time T=0 sec I close switch S1 and the pin 3 goes high.
    2. At time T=10 sec the pin 3 goes low.

    Now even if i close the switch S1 between time T=0 to time T=10, the output at pin 3 will still go low.

    What i want is that if i close the switch S1 at time T=5 seconds, the timer should start again and output should remain high till time T=15 seconds.

    Basically i want a time delay switch, which should activate immediately, but time out after time "T" which is determined from time of last trigger.

    can i use simple RC network connected to a transistor switch where the swith will close immediately on giving the pulse, but will open again after a time delay determined by RC network. and if i give a trigger the circuit again in between the time delay, the time counting should start all over again...

    can you please help....

    Attached Files:

  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Uhhh, no it won't. This is not a retriggerable design, but you do have a major design flaw, in that you don't have a signal conditioner. I did explain this in my article, which I referred you to last series of posts.

    http://forum.allaboutcircuits.com/showthread.php?t=24640

    Try reading it, the design is the same, and it gives a thorough explanation.

    555 Monostable

    To recap, the input can not stay low beyond the timeout, it is an illegal condition. It will ignore any retriggers while in timeout. A 0.1µF cap on pin 5 will suffice.
  3. devalvyas

    devalvyas Thread Starter Active Member

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    Yes sorry i omited the signal conditioner...actually i was trying to re-trigger the circuit during the timeing cycle and hence tried to omit the signal conditioner....my actual circuits does contain signal conditioners..
  4. devalvyas

    devalvyas Thread Starter Active Member

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    Other way round...can i make any arrangement at the input where the pin 2 receives input trigger signal only if the switch S is closed for time T?

    Normally the moment I close the switch S, pin 2 receives the input trigger.

    What I would like to do is that the Pin - 2 should receive an input trigger only after I press the switch S say for eg. 2 seconds. If i press the switch S only for one second and then open it, 555 should not be triggered.

    (i am attaching the schematics with signal conditioner...)

    Attached Files:

  5. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    I'm not sure what you want. If I read you correctly then you need a second 555 and a AND function (which can be a couple of diodes).

    There is another way, but it is a bit more complex. A simple counter that times the duration of the time the switch is pressed, when it exceeds this number it sends a signal to the monostable. This will rapidly escilate the number of parts, I don't really recommend it.

    IF I understand what you're wanting it is a simple modification from the other thread.
  6. devalvyas

    devalvyas Thread Starter Active Member

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    I have tried the following circuit for a power off delay switch.

    When i connect the switch S2, the transistor switch is closed there by turning the LED1 on.

    when i open the switch, the LED1 does not switch off immediately, but switches off after some time, T

    Even if i close the switch on S2, again, the time T will start from that point of contact.

    but i am facing two problems.

    1. the led does not entirely turn off for a long time after i open S2. I want a pulse action from transistor Q1, ON-OFF , like a square pulse, how do it do that?

    2. how accurately can i control the time delay here? can i bring it down to 0.02 seconds?

    thanks

    Attached Files:

  7. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Check my article on 555 Schmitt Trigger. Any IC Schmitt Trigger will work, but this is easy access. Add that to your schematic and you have a retriggerable monostable.

    You can bring the time down to anything, but at 0.02 seconds your finger will be the slow component in the loop.

    BTW, we are now in the realm of debouncing switches. A typical switch makes/breaks several times when it switches, and digital logic sees all, responds to all.

    [​IMG]
  8. RiJoRI

    RiJoRI Well-Known Member

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    "What i want is that if i close the switch S1 at time T=5 seconds, the timer should start again and output should remain high till time T=15 seconds."

    This is called a retriggerable monostable timer.
    Have you looked at the 4098 or the 74HC423?

    "What I would like to do is that the Pin - 2 should receive an input trigger only after I press the switch S say for eg. 2 seconds. If i press the switch S only for one second and then open it, 555 should not be triggered."

    I'd look into using a tiny microcontroller for this effect.

    --Rich
  9. devalvyas

    devalvyas Thread Starter Active Member

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    i think a re-triggerable monostable is what i am wanting...will read more about it..i have downloaded the data sheet for CD4098B from Texas instruments. is there any link on web which explains the working in details...just like Bills Marden's book on 555 timers..?

    I will need to read on the schmitt trigger also. will read and post the reply.

    My last thread on the twin 555 circuit is a different requirement than what i require in this thread...


    Thanks a lot...wish some day i too will be able go give such guidance to guys like me who need it....:)
  10. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    You'll find these designs respond to block diagrams very well. I'm still not sure how you're drawing this, Word seems most unlikely. I use M/S Paint myself, and have a batch of templates you can cut and paste from, as well as a lot of reference data in my blog in my personal introduction.

    Bill's Index
  11. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    OK, here's how I would do it. The same formula will work for both sections, I've set the first for 2 seconds.

    I've also included the crossover information to use a 556, a dual 555 in a single chip.

    [​IMG]

    Attached Files:

    Last edited: Jun 19, 2009
  12. devalvyas

    devalvyas Thread Starter Active Member

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    Ok will buy a schmitt trigger ic and try it out. But yesterday i bought a 4098 re-triggerable monostable chip. i am attaching the datasheet and the schematics that i have put to gether.

    something seems to be wrong as i am not getting any out put pulse.

    i am using a 4.5 to 7 volt power supply. its basically ac to dc converter with around 500 mA capacity.

    can you pl advice on the schematics. its not working...

    Attached Files:

  13. devalvyas

    devalvyas Thread Starter Active Member

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    I am reattaching the schematics, i had put the capacitor wrong. The values were on higher side. 4098 does not allow more than 100 micro Farad cap. but i am facing a different problem.

    I am triggering the input by connecting pin 12 (+Tr2) to +Vcc. Now if i connect if for a short duration the output does not change. the output at pin 9 and 10 only change after i connect the pin 12 to +Vcc for time equal to the time set by R and C, Where T=0.5 RC

    I mean it should be other way round. The output should change immediately on my connecting pin 12 to +Vcc. It should at that level for T= 0.5RC and from that level it should change back after the T=0.5RC.

    Also, when i switch the power on, the output at pin 10 (Q2) is showing High, ie. the LED is glowing and out put at pin 9 (-Q2) is showing low, ie the led is not glowing. The moment i connect the pin 12 to +Vcc, nothing happens for a short time. The output changes i.e. the LED change only if i keep the pin 12 connected to +Vcc for time T= 0.5RC. The moment I disconnect the pin 12 from +Vcc the output immediately changes.


    Why is the behaving the other way round? is there something wrong in the way i am triggering or something wrong in my understanding...

    What i want is that the output at pin 10 should change immediately on my applying trigger by connecting pin 12 to +Vcc for a short time. Ie. it should be positive going pulse trigger. Output at Pin 10 should stay at that level for T= 0.5RC and then change back. In case i again trigger the circuit by connecting pin 12 to +Vcc, the time cycle should start again.

    Attached Files:

  14. Bernard

    Bernard Senior Member

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    Apparently this is a repeat now, but why waste a crude drawing. RC about = to time. IC is comparator. Output pulse extended eack time switch is closed.

    Attached Files:

  15. devalvyas

    devalvyas Thread Starter Active Member

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    i will try out the comparator circuit by Brenard...but since i have already started on 4098 i would like to get the same output with it, but i am not getting it right...i made these modifications in the circuit, but still when i swith on the power, the output Q goes high and if i give small input pulse to input +Tr, nothing happens. only if i connect the input swith to +Vcc or Ground for time more than T=0.5RC, the out put goes low.

    The output again goes hi the moment i disconnect the input to +Tr.

    I also tried to measure the value of voltage at +Tr and I was amazed to find the voltage at input terminal +Tr was showing +2.3 volts...

    can you please guide me on a proper circuit for 4098 and if i have to keep any thing in mind to make it work...

    thanks

    Attached Files:

    Last edited: Jun 21, 2009
  16. devalvyas

    devalvyas Thread Starter Active Member

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    I have tried this circuit.
    i am attaching the circuit and the component values.

    but when i give the input trigger, i dont get the output pulse immediately, i get the pulse after some delay. ofcource when i open the switch, i get pulse dies after some time which is fine as per the time delay.

    why is the output pulse not immediate?

    Attached Files:

  17. devalvyas

    devalvyas Thread Starter Active Member

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    After some debugging i finally got it right by putting couple of diodes...attaching the schematics...let me know if i need any thing more in it...

    Attached Files:

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