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How to auto trigger a 555 timer

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abuhafss

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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?
 

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FvM

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To cascade the timers as intended, you need a timer with an edge trigger feature. The 555 basic circuit doesn't provide it, but you can easily add it to it's trigger input by an additional RC circuit, a differentiator that triggers the 555 on a falling pulse edge.
 

abuhafss

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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.
 

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FvM

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Yes, it looks very good. Personally I would supply a small series resistor (e.g. 10k) for the 1M potentiometer, otherwise turning it to minimum resistance causes a short at the 555 discharge output.
 

abuhafss

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Yes, it looks very good. Personally I would supply a small series resistor (e.g. 10k) for the 1M potentiometer, otherwise turning it to minimum resistance causes a short at the 555 discharge output.

Thanks.
By the way, one friend has advised me that "15 second period generated with a 555 is not likely to be very stable" so I should go for ICM7242 or for some PIC. What do you say?
 

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As good as it looks, the whole circuit may behave unpredictably on power on, that is before the first cycle is initiated ..
I would do it with just one microcontroller, and PICAXE would by (my choice) a good candidate for the job ..

For example, instruction (in PICAXE BASIC) WAIT 15 equals to 15 sec delay ..
So the sequence:
WAIT 5
WAIT 15
WAIT 5
gives you 5s, 15s followed by another 5s, and so on ..

IanP
:grin:
 

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abuhafss

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I have been playing with the BASIC language about 20 years back. But, have no experience of using PICAXEs.

Just now, learnt a bit about them. But could not understand how to connect relays, LEDs. Some relays are to switch 220v. Can you recommend me some site where I can learn easily.

Thanks.
 

FvM

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Generally I took as granted that you want a design 555 timer circuit. Crystal based timers, microprocessors are a different option.
15 second period generated with a 555 is not likely to be very stable
Using good capacitors, it can be very stable (Within in the bounds of 555 general accuracy, see above)
the whole circuit may behave unpredictably on power on
Good point. Can be overcome by feeding a reset to all 555 timers.
 

abuhafss

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Generally I took as granted that you want a design 555 timer circuit. Crystal based timers, microprocessors are a different option.

Using good capacitors, it can be very stable (Within in the bounds of 555 general accuracy, see above)

Good point. Can be overcome by feeding a reset to all 555 timers.

Thanks for explaining the difference. Now I am relaxed.

1. Could you please clarify what do you mean by "good capacitors" and "within in the bounds of 555 general accuracy"?
2. How shall I feed a reset to each timer?
3. For the 10K res in series with the 1M potentiometer ------> You mean 10K connected to +15vDC and then the 1M? Or the other way round?
4. Just for information, and to have better suggestions from you, all the 4 relays are to switch 220v devices.
 

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IanP

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I have been playing with the BASIC language about 20 years back. But, have no experience of using PICAXEs.

Just now, learnt a bit about them. But could not understand how to connect relays, LEDs. Some relays are to switch 220v. Can you recommend me some site where I can learn easily.

Thanks.

Free software:
PICAXE

Manuals (accessible through software - HELP, or can be separately downloaded):
The PICAXE manual is divided into three separate sections:
Section 1 - Getting Started (picaxe_manual1.pdf)
Section 2 - BASIC Commands (picaxe_manual2.pdf)
Section 3 - Microcontroller interfacing circuits (picaxe_manual3.pdf)
This third section provides general microcontroller interfacing circuits, and
Example programs, for most common input/output transducers used within
microcontroller circuits.

https://www.rev-ed.co.uk/docs/picaxe_manual2.pdf
https://www.rev-ed.co.uk/docs/picaxe_manual3.pdf

IanP
:grin:
 

FvM

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1. Could you please clarify what do you mean by "good capacitors" and "within in the bounds of 555 general accuracy"?
2. How shall I feed a reset to each timer?
3. For the 10K res in series with the 1M potentiometer ------> You mean 10K connected to +15vDC and then the 1M? Or the other way round?
4. Just for information, and to have better suggestions from you, all the 4 relays are to switch 220v devices.
1. Non electrolytic. For specified accuray, see a 555 data sheet.
2. They have a reset input. You can connect an RC circuit to all in parallel, it's time constant should be longer than of the negative edge RC circuits.
3. By ohms law, the order shouldn't matter, I think.
4. It may be necessary to provide RC snubbers for 230 V contacts to reduce contact arcing.
 

abuhafss

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1. Non electrolytic. For specified accuray, see a 555 data sheet.
2. They have a reset input. You can connect an RC circuit to all in parallel, it's time constant should be longer than of the negative edge RC circuits.
3. By ohms law, the order shouldn't matter, I think.
4. It may be necessary to provide RC snubbers for 230 V contacts to reduce contact arcing.

Can you guide me how to work out the values for R & C for the snubbers?
 

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