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How to make an LED blink using a 555 timer? Answered

so im doing a school project and i need to figure out this whole circuit thing. my teacher recommended me this circuit (picture related) but i dont understand it. can anyone help me?
i am trying to do this on a bread board (i think they're called that). i am making LEDs that are blinking using a 555 timer and a BD139 transistor. and some resistors that i dont know the resistance of.


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Jack A Lopez
Jack A Lopez

4 years ago

There is a picture, a block diagram, attached to the Wikipedia article for "555 timer IC".


It is a block diagram, like you'd find in any datasheet for the 555, except the author has added some colors, to sort of neatly define the internal parts of the 555 as exactly six colored blocks.

And here's the link to the rest of the article, if you're interested in that sort of thing,

Anyway, regarding the block diagram picture, I think it looks very pretty, and it makes perfect sense to me. It tells me, in detail, as much detail as I want to know, about how the 555 timer works. It's just a total, and perfect explanation.

I am not sure what that picture looks like to you.

It may be the case that this picture looks like an alien language, like hieroglyphics, just cluttered collection of symbols, but you don't yet know what the symbols mean.

If you don't speak French, and you listen to two people having a conversation in French, you can tell which person is speaking, and you can hear words, and you're pretty sure the words have meaning, and these French speakers are saying meaningful things, but it's like the hieroglyphics. You can't decode it, and you don't know what's being said.

So, you know, one approach is to record the whole conversation. Then go over it, word by word, with your French-to-English dictionary, and in this manner, decode what is being said. If you listened to, and word by word translated several hundred, or maybe thousands, of hours of this, you might actually learn to speak French.

Similarly, you can look up the symbols in the block diagram. What is a "voltage divider"? Ah ha! A voltage divider does this. What is a "comparator"? A comparator does this. How does the flip-flop work? What does it do? How hard could it be, right? It's only like six effing things!

You know, if you went over all this enough times, eventually you'd puzzle it out, just like you might learn how to speak French if you spent enough time studying the grammar.

But maybe you don't have the luxury of large amounts of time? Yeah. You mentioned it's for a school project. Presumably, you need to cobble something together before the due date, whenever that is.

Well, if you need to say something in French, without actually knowing how to speak French, words, spelling, grammar, etc, the solution, is to use a book of common phrases.

Suppose you need to use the bathroom? Don't fret. That's on page 1 of any good phrase book:

Qu sont les toilettes?

(oo sahn lay twah-leht)

Where are the bathrooms?

You see that line in the middle in parenthesis? It's spelled phonetically, in English, so you don't even have to know how to read French to sort-of pronounce it right. How slick is that?

By the way, I don't speak French. I got that handy phrase from this page.

It works the same for any other language, Spanish, German, Arabic, Klingon, whatever. You just arm yourself with the phrasebook for that language, and you say all kinds of useful stuff, like, "Hello!", "What time is it?", "My name is..."

So, if you've found the patience to read this far, by now you probably wondering:

What is the equivalent to a phrasebook for the 555 timer IC?

The answer is: the example application circuits in the datasheet.

Actually it works the same for any IC. Every datasheet has at least one example application circuit. It has to. It's like a law of nature. It's like an unbroken series of Catholic Popes, and bears that continue to defecate in the woods.

Anyway, allow me to explain how this actually works. This is how the pros do it, step-by-step:
Step 1: Fetch the datasheet.
This place,
has most of them, including LM555, or NE555, or TLC555, or whichever one you've got.

Step 2: Look in the datasheet, and find the section labeled "Applications", or "example applications", or just look at the pictures, for pictures of circuit diagrams.

Step 3: Find the example application that looks like it will sort of do what you want it to do. This is analagous to looking in the phrasebook for a phrase that says the thing you want to say. Hopefully you'll find something close enough.

Step 4: The example circuit might not be as easy as saying, "Bonjour.(bohn-zhoor)" In fact there might be several component values that you have to choose, C, Ra, Rb, etc. Well, it's sort of like the phrasebook can tell you how to say, "My name is...", but the phrasebook cannot tell you what your own name is.

In the LM555 datasheet, published by National(r), the datasheet I am looking at,
the circuit for making the output oscillate; i.e turn on, then off, then on, then off, then on, then off, etc, is called "ASTABLE OPERATION". Sorry for screaming that in all caps, but that's the way that heading is written in the datasheet, on Page 7, the circuit itself is Figure 4.

You might have to do some math, to figure out values for your capacitor, resistor a, resistor b, and so on. You want to be an engineer, amirite? Yeah. You can do math. It might hurt a little, but it's something you'll get used to.


Step 5: Sometimes, just sometimes, the datasheet is not totally clear, not easy to follow. Almost as if the author of the datasheet writes in a foreign language only he or she understands, if that sort of thing is even possible.

I know I promised it was going to be as easy as just copying one of the datasheet's easy application examples, and sometimes it is. But if the datasheet is confusing, then you proceed to Step 6.

Step 6: Look for circuit examples people have published on the web. Because circuit diagrams are images, use an image search, like Google(r) Image search. For the 555 timer IC, you'll probably only get a few billion hits.

Step 7: Look in the "related" panel, on the right there. What do we see there? More 555 circuit related pages than you can shake a proverbial stick at.

Step 8: By now you should be getting really, really close to building a working circuit, even if you're blindly copying someone else's circuit, that you found, wherever. And that will totally work for an assignment like this. I am guessing you just have to put the thing together with your own hands, or simulate it with your own typing and mouseclicks, in the event you're not actually building it IRL.

I dunno what Steps 9 and 10 are, but I am really hoping you figure this out, and that you don't have to bring it back to the Answers forum. I mean you've got smart friends in real life, right? Or you've got a teacher. Teachers are kind of like friends, kind of...



4 years ago

Ill help if you explain the difference between NPN & PNP ??

With symbols and label names like gate and source etc


Answer 4 years ago

In a nutshell, this is the classic astable circuit that uses 2 resistors to set the charge and discharge rate of 1 capacitor. When the 555 "detects" the capacitor is discharged below 1/3rd of supply voltage, it uses one of the resistors to slowly charge it up until it reaches 2/3rds of supply voltage, then it flips to discharging the capacitor back to 1/3rds of supply voltage with the other resistor. To examine how it does that, look at the internal circuitry, and learn about basic astable oscillators and hyterisis.


4 years ago

In the time it took you to create two useless double postings you could have found the answers using Google.
It is a school project which means you should do the research on it.
In your diagram nothing is labelled so no chance to give any decent cooment on that.
Start by checking the datasheet for the NE555 and by the examples in there your circuit chould become much clearer.
There are also plenty of websites with examples on blinking with a 555 and a lot of tutorials explaining in deatial how the example circuit actually functions.
If you don't use copy and paste but write in your own words your techer should be happy.

And if you can then please delete or close the other topic ;)