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Old 06-27-2012, 11:21 PM
ThePsuedoMonkey ThePsuedoMonkey is offline
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Default LED cascade: sans 555/4017

Hello everyone! I have been trying to create an LED cascade circuit without using the 555 or 4017 integrated circuits. The best design that seemed to work in the Falstad simulator can be seen here (uses java). Essentially it's a few NPN transistors with a base-emitter daisy chain,each with 10kOhm resistors above the reference voltage, and the LEDs in series with the collectors. When I tried that circuit in my test board, nothing seemed to get it to work. I've seen a lot of info about the aforementioned ICs, but very little that doesn't use them. Am I being silly for attempting this, or am I making it too complicated?
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Old 06-28-2012, 03:46 AM
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KJ6EAD KJ6EAD is offline
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That RC bucket brigade approach can be difficult to balance. It's a good teaching example to show the advantages of digital logic over analog circuitry.

You can replace the 555 clock with one made from a 3 inverter oscillator or if you want to stick with the discrete approach, a 2 transistor/2 capacitor astable. Separating the clock function from the driver functions has some advantage in consistent timing but then you need lots of latches to replicate the counter function of the 4017.

You could use a smaller counter (4 bit) if you only need 3 outputs or you could use a shift register but then you'd have to include a method to load a starting value.

I get the impression that you want to do this using only discretes but have performance similar to the digital logic method, no matter the part count. Is this a school or science fair type project idea or what?

Take a look at using an astable oscillator followed by a 4-bit ripple counter, all made from discrete components.

Last edited by KJ6EAD; 06-28-2012 at 03:58 AM.
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Old 06-28-2012, 07:53 AM
ThePsuedoMonkey ThePsuedoMonkey is offline
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Nah, this isn't for anything as fancy as all that: just me trying to do a little mood lighting. You'd be right about me trying to only use discrete components; I tend to shy away from components that I don't fully understand. Plus they seem like cheating. The astable oscillator definitely seems like a good start. I tried building a ripple counter out of bi-stable multivibrators, but can't think of a way to get them to nest properly.

I did have a little victory with the daisy-chain method though: using the negative of the LEDs as the base terminal lets you string a whopping four of them together if you start the chain with a darlington pair. Not very impressive though.
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Old 06-28-2012, 02:23 PM
wayneh wayneh is offline
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Originally Posted by ThePsuedoMonkey View Post
...I tend to shy away from components that I don't fully understand....
Everyone does. But learn to embrace datasheets. With enough motivation, a datasheet, and a few words of wisdom from experts around here, you can come to "fully understand" things that were once an utter mystery. To me that's one of the most satisfying aspects of any hobby, learning a new tool.
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Old 06-28-2012, 06:53 PM
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Bill_Marsden Bill_Marsden is offline
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LEDs, 555s, Flashers, and Light Chasers

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You can do it simply, or your can do it complex.
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
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