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Flashing question

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techno
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Username: techno

Post Number: 64
Registered: 07-2005

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Posted on Thursday, 26 June, 2008 - 08:26 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

iam faced with a small problem i need to build a circuit that will allow 6 white leds to flash at different times any suggestions?i dont know wheather a 555 based circuit will work or a 555 circuit coupled with a 4017 decade counter thanks techno
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techno
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Post Number: 65
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Posted on Thursday, 26 June, 2008 - 08:29 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

i fogot to mention guys it must be quick flashes thanks techno
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grab
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Post Number: 763
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Posted on Thursday, 26 June, 2008 - 10:18 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Speed of flashing isn't a problem - any electronic circuit from the 555 upwards will be able to flash the LEDs at speeds well past kilohertz (ie. faster than your eye could see), so no problems there.

But "flash at different times" covers a lot of different ways this could flash. More info needed! :-)

For example, do you want each LED to flash at different times from each other, but at a predictable rate? Or do you want them all to flash at the same time, but in a sequence that isn't just on half the time, off half the time? Or do you want them all to flash at the same time, but in a random unpredictable sequence?

Like I say, more info needed!

Graham.
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techno
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Post Number: 66
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Posted on Thursday, 26 June, 2008 - 10:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

grab i want each led to flash at different times from each other at a predictable rate thanks mate
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techno
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Post Number: 67
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Posted on Thursday, 26 June, 2008 - 10:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

grab iam designing a scene for a project the flashing leds are to act as camera flashes . so basically it must be like 1 led must give a quick bright flash then go off then few seconds later another led must give a quick flash and go of the sequence must go thru 6 leds and then start the sequence again
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vlf
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Post Number: 166
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Posted on Thursday, 26 June, 2008 - 11:38 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Techno; sounds like a 555 timer & 4017 devide by-ten. This will as you have said, produce sequence On-Off-On-Off to each 6 LEDs one at a time... you need to loop-back the seventh output to the reset pin 15; this will continue the loop sequence while the 555 clocks at the speed you have set it's frequency at.

However, if you are thinking of a random light sequence, this may take far more work, and best using a PIC via programming, although it's possible by normal electronic design, but needs a random "Noise" source to alter the sequence or the timer; possible to use pin 4 of 555 reset, via another 555 as a Mono-Stable, with the Noise source triggering the Mono-Stable; this is not tried in practice but, it could work.

URL site provides more info:

http://www.elecfree.com/electronic/running-lights-by-lm555-4017/

and even more info:

http://members.shaw.ca/roma/thirty-four.html

Regards, Brian.
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techno
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Post Number: 69
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Posted on Thursday, 26 June, 2008 - 12:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

thanks vlf .techno
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ant
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Post Number: 300
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Posted on Thursday, 26 June, 2008 - 08:27 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

Is a PIC the best way here? I did a similar thing a long while ago for a doll's house, much slower cycling obviously but still apparently random and all different. I just used five 7555 (well, 555 at the time!) timers and tweaked the pots to give a pleasing effect.

But then, I'm a dinosaur!

Regards Ant
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eagre
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Post Number: 267
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Posted on Friday, 27 June, 2008 - 02:22 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For real flexibility in timing I think a PIC, or other MCU, is the answer. Any with six outputs will do (e.g. any 14 pin, 18 pin or larger PIC). If you don't need more than 25 mA per LED the PIC can drive them - otherwise provide transistor drivers. The circuit will be very simple with few parts; you control it with great flexibility in the software.

Just establish an outer loop with a counter, cycling at a rate much faster than LED on/off periods. Then set constants to specify the various LED periods and convert these to on/off counts.

If you are averse to assembly programming this can easily be done in BASIC using suitable PicAxe or other BASIC-programmable MCUs.

Ed
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grab
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Post Number: 765
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Posted on Friday, 27 June, 2008 - 10:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A PIC is the least-components solution. But if Techno needs to ask how to do this, I'd guess he's not yet got the experience to be a-PICin' and a-grinnin'. ;-)

As VLF said, 555 and 4017 will be the easiest.

You're going to need transistors on the outputs to drive the LEDs, because logic chips can't deliver enough current. The usual way is to have one NPN transitor per LED connected to 0V, and permanently connect the LED positive to +V via a resistor.

You do have a further problem, which is how to make the flash happen. The 4017 will hold a level until it gets a new value, so the basic circuit will make each LED stay on in turn. I can see two possible solutions to this. One is to put an edge-triggered monostable (eg. one half of a 4047) on each output, and set the flash time you want for each LED. A more cunning way would be to make the 555 pulse set the flash time, and as well as feeding the 555 to the 4017, you also feed it to something controlling the +V side of the LEDs. Then the currently-selected LED will only turn on when the 555 pulse is high, and it'll be turned off when the 555 pulse goes low. A relay or a power driver chip would do for that. Personally I'd favour using three 4047s, which has the advantage that you can make each LED flash time different - that might look better on stage. It's also a little simpler if you're not used to combining relays and digital logic.

Graham.
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thomas
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Post Number: 246
Registered: 04-2005


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Posted on Friday, 04 July, 2008 - 08:39 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Techno,

Here's a simple method, although it's not clear what you mean by flashing an l.e.d. at "different times". Use six inverter gates of a CMOS 40106. Let's take the first gate. The output goes to a cap (say 10µ) in series with an l.e.d. and perhaps a ballast resistor, wired between the output and either +VE or 0V. When the state of the output changes, the l.e.d. flashes. But feed that output also via a resistor to a cap at the input of the next gate (the cap wired between the input and 0V). Now add a similar l.e.d. arrangement to the next output, and so on. I think you should be able to set up a circular circuit -- six gates in a circle. If not, you could operate the first gate with a simple timer -- or use one of the six gates as a clock generator. An advantage of this circuit is you can time each pause between flashes, so as to e.g. speed up the sequence as it nears the end. Hm, maybe I should write this up for IU.

With kind regards,
Thomas.

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