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  #1  
Old 06-19-2009, 07:03 AM
devalvyas devalvyas is offline
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Default simple time delay switch- 555 or simple rc network with transistor

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....
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File Type: doc monostable 555.doc (25.0 KB, 115 views)
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  #2  
Old 06-19-2009, 07:15 AM
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Bill_Marsden Bill_Marsden is offline
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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/sh...ad.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 0.1F cap on pin 5 will suffice.
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Old 06-19-2009, 10:55 AM
devalvyas devalvyas is offline
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Quote:
Originally Posted by Bill_Marsden View Post
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/sh...ad.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 0.1F cap on pin 5 will suffice.
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..
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Old 06-19-2009, 11:16 AM
devalvyas devalvyas is offline
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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...)
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File Type: doc monostable 555.doc (25.5 KB, 30 views)
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  #5  
Old 06-19-2009, 12:36 PM
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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.
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  #6  
Old 06-19-2009, 12:54 PM
devalvyas devalvyas is offline
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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
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  #7  
Old 06-19-2009, 12:58 PM
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Bill_Marsden Bill_Marsden is offline
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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.

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  #8  
Old 06-19-2009, 01:35 PM
RiJoRI RiJoRI is offline
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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
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  #9  
Old 06-19-2009, 03:57 PM
devalvyas devalvyas is offline
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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....
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  #10  
Old 06-19-2009, 05:04 PM
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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.

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