# Where I can learn about 555 Astables.

Discussion in 'General Electronics Discussion' started by MstrKurt, Oct 5, 2012.

1. ### MstrKurt

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Jun 13, 2011
Hey guys, does anyone know where would be a good place to check for all the information I could need for designing/building and testing an astable with a 555 IC?.

I have purchased a book from amazon called Ic 555 Projects, but would like to do some reading while I wait on the book.

Thanks guys.

25,448
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Jan 21, 2010
3. ### MstrKurt

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Jun 13, 2011
I'm just asking for peoples opinions on the best electronic information websites. I'm quite aware that Google can answer most questions. Also, I don't mean to sound disrespectful but what is the purpose of these forums if you're just going to link people to Google?.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Because for such a general topic that is covered so well in so many places on the web, why do you expect us to cover it again in a way that's probably not as good.

If you look at these, spend a little of your time doing some reading and thinking, and still are confused about some aspect, THEN we can help by providing some explanation tailored for that point. If we were to explain fully the working of the 555 every time we were asked, you would find hundreds of such explanations on this forum.

It doesn't mean we're not happy to help.

5. ### MstrKurt

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Jun 13, 2011
Ok, that's a fair point. My apologies.

After a little messing around on Electronics Workbench and searching, I have come to understand that in a 555 Astable Multivibrator circuit R1 control how fast it takes C1 to charge?. And R2 controls how long it takes before C1 begins to charge.

Can anyone confirm whether I'm right or not?. I'm still struggling to understand what impact the size of C1 has on the circuit. I assume that the IC recognizes that when the capacitor begins charging it should produce a high input for as long as the capacitor continues charging, upon discharge it makes the output low?. Thus giving the mark-space ratio?.

I have an attached picture of the simulation to help you see the circuit so you know where R1, R2 etc are.

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6. ### CiaranM

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May 19, 2012
ha ha you got LMGTFY'd. I suggest thatyou get a book entitled 'Practical Oscillator Circuits'. It describes the 555 and 556 ICs and how they work, plus it has many great circuits.

http://homepage.ntlworld.com/g.knott/elect388.htm
sometimes looking at the circuit diagram can help. this is the 555 configured as a monostable, so its different to what you're interested in, in terms of output; but it'll still be helpful to look at.
observe the capacitor at the bottom left and forget about the rest of the circuit. the cap charges up to 2/3 of the supply (this is a RC circuit as there are resistors from the supply to the cap). when this V is reached, the discharge transistor is switched on, so the cap starts to discharge. when the V across the cap reaches 1/3 of the supply, the discharge transistor is switched off and the cycle repeats ad infinitum.

I can't explain it that well; you should really get the book!

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
OK, the resistors are not labelled, so I'll call the 50k resistor R1 and the 10k resistor R2.

There are several things happening in this part of a 555 circuit.

The first thing is the top of the capacitor is connected to Trigger and Threshold. These inputs simply detect when the capacitor voltage rises above 2/3 Vcc or below 1/3 Vcc. When it voltage rises above 2/3 Vcc, Discharge is pulled low until the voltage on the capacitor falls below 1/3 Vcc.

So look at the circuit again.

When Discharge is not pulled low (it is effectively open circuit) then the capacitor charges through R1 and R2).

When Discharge is pulled low, the capacitor is discharged vis R2 and the discharge pin to ground.

So both R1 and R2 control the speed at which the capacitor charges, and R2 alone controls the rate it discharges.

8. ### MstrKurt

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Jun 13, 2011
Ok thanks for the help guys .

9. ### CiaranM

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May 19, 2012
the equation for (astable) frequency is 1/(0.7 x (R1 + 2.R2) x C1)

Say that R1 is 5k, R2 is 5k, and C1 is 20n.
Freq = 4761.904761Hz.

Say that C is increased to 820n.
Freq = 116.1440186Hz.

Note that a larger capacitance decreases the frequency, and vice versa. This because a cap with a larger value takes more time to charge and discharge.

--

you're welcome!

10. ### MstrKurt

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Jun 13, 2011
Would anyone know how I would calculate what value components are needed for a specific spec?. I've looked on google but i'm unable to find an answer.

11. ### gorgon

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Jun 6, 2011
Last edited: Oct 8, 2012
12. ### MstrKurt

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Jun 13, 2011
I'm still unsure how the data sheets tell me what component values can be calculated for a specific specification.

Can transposition formula be used to do this?.

Last edited: Oct 8, 2012
13. ### gorgon

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Jun 6, 2011
You need to define the specifications of what you want to get out of the design. Like the frequency and duty cycle for an oscillator. Then you look at the timing diagrams to get a ballpark value of the capacitor, for example. From that you can calculate the resistors, using the formulaes in the datasheet.

If your ballpark value was far out, adjust it and calculate again. it is a kind of iterative process.
For final tuning the values to standard component values, you may need to add components, in series or parallel. Or add a pot for tuning.

This is the art of guesstimating

TOK

14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Also remember that for a particular frequency (and duty cycle) there is not just a single set of values. You can use larger capacitors and smaller resistors to get the same effect (and vice versa).

The specs will tell you about maximum and minimum values of resistors and capacitors in some cases (or it may leave you to figure it out from raw data).

15. ### CiaranM

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May 19, 2012
what is the 'spec' you speak of?
if it is, for example, 555 operating temperature, then read the datasheet. If you want the frequency equation rearranged to give, say, R1 or C, then ask me.

16. ### gorgon

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Jun 6, 2011
The specifications we talk about is the specifications for the end product, what you want to get out of your design. It is normal to setup the system specs when you design a new device or product. If you don't do that, how will you know how to design anything, or know that you are finished with what you wanted. You need some guidlines to start with, and to use as testcriteria underway.

TOK

17. ### CiaranM

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May 19, 2012
ahhh, those sort of specifications. thanks for clearing that one up

18. ### gorgon

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Jun 6, 2011
But, you'll also need the datasheet's technical specs of the part in question, to find if this is the correct chosen part in this particular design.

So, we are in fact talking about several 'specs' in all cases of designing gizmos, of any kind.

TOK

19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Yeah, I was talking about the specs for the 555 (the datasheet).

Lets say you want a particular frequency and you decide on resistors of 10k, and 1k, and a capacitance of 0.1uF. You (in a perfect world) get the same result if you multiply the resistor values by some constant and divide the capacitor by the same constant.

This means that 100k, 10k, and 0.01uF would be the same, as would 1k, 100R, and 1uF.

However you can't take that too far. 1M, 100k, and 0.001uF might not quite behave the same due to leakage and input currents.

Likewise, 100R, 10R, and 10uF would also behave differently because of limitations in the discharge path (indeed, going too far along this path will damage the 555).

So once you use some calculator to determine the resistor values (or go by the formulae in the datasheet) you need to look further into the datasheet to determine if these values are in an acceptable range. Keeping the resistors between a couple of hundred ohms and a couple of hundred k is a reasonable rule of thumb.