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# pin 7 not used?

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#### daviddoria

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http://www.bandtank.com/~calculusap/leds.gif

in all the tutorials i read on 555s, the resistor between 6 and 7 (usually called R2) is very important. on this schematic there is not even a pin 7. why is this?? i thought i was understanding then i saw this hehe.

david

Hi David. Not to worry. The schematic you have (I think) is only showing how to take the output from some random 555 and feed a counter - it assumes you have the 555 running already. So, connect pin 7 to + thru a 1k resistor. Connect pin 2 to 6. connect pin 5 to ground thru a 0.01 mfd disc cap. Adjust the pulse timing (at pin 3) of your 555 with a resistor between pins 6 and 7.

The idea (I think) is to get a 555 running at a pulse pace that you like first . . . then use this circuit to count the pulses up to ten.

gene, i understood all that except for the resistor between 6 and 7. i see nothing there.

let me give my explanation of a 555 and see if it is correct.

1
connect straight to ground

2
connect to 6 (this is the astable part, making it restart itsself)

3
output (the pulse you are generating)

4
since we are running this astable, connect to Vs

5
the cap (.01uF) between 5 and ground is for stability purposes

6
connect to 2 (making it reset, however i don't understand the pin "title" of "threshold"

7
through a 1k resistor to Vs (purpose?)

8
Vs to the IC

ok if that is all right..

what is the resistor between 3 and 6 doing in the schematic i provided?

if we clear all that up i should be able to start making this!
yay

david

Sure. Almost there. I assume you are going to establish your pulse in an R-C (resistive - capacitive) circuit rather than crystal control. To do this there are three elements added to the 555 that establish pulse frequency.

No. 1 - (R1) - this is the resistor you have mentioned going from pin 7 to + (I like 1k)

No. 2 - (C) select an electrolytic capacitor to go from the union of pins 6 and 2 to the ground (I like 10 mfd).

No. 3 - (R2) - The resistor (or series of resistors) going from pin 6 to 7. This, more than anyother element establishes the pulse rate. Keep this element between 10k and 470k. A good place to start is 68k. If you want to predict the outcome, the following is a shortcut (i.e., leaves out some stuff but gets you close):
freq (Hz) = 1.44/((R1 + (2 x R2)) x C)
1.44 / ((1000 + (2 x 68,000)) x 0.00001)
1.44 / (137,000 x 0.00001)
1.44 / 1.37
1.1 Hz

This is an interesting area of IC manipulation that is all over the internet.

three questions:

1
you say the 3rd thing for timing is the resistors between 6 and 7. are those necessary?

2
second on this schematic it has 47k between 3 and 6
what does that do?

3
i read that pin 5 is for frequency modulation. (dont know what that is, but i dont think i'm doing it ). can i just not have anything connected to pin 5 since i'm not using it? or should i put it through a .01uF cap to ground?

Hope I don't get a double post - my connection dropped in the middle of typing.

I think your confusion on the 555 stems from your schematic. The circuit you are using is not a good 555 circuit - it is OK for attaching a 4017 to a 555 that is already built and running. I suggest you do a search from your web browser "555 circuit" for tons of good info.

Here's one:

If you move down the page to astable operations you will see a good schematic for a 555. Also there is a little pulse calculator for you to use. You will see that if you use the values I suggested (R1 = 1k, R2=68k, C=10 mfd) you will end up with a 1 Hz pulse in a 50% duty cycle. The sample schematic should answer your questions. As you proceed, add a LED and dropping resistor between pin 3 and ground so you can watch the action.

To answer your first question about the necessity for R2, go to the calculator on the web page and simply raise the value towards infinity (no resistor) and see what happens. Remember I said earlier that this was "the most important element?" You might also put a 100k variable resistor in here so you can watch the pulse time change.

Question 2 - Please accept that yours is NOT a schematic for building a 555 in astable configuration - it is for a 4017. You can easily select a good 555 circuit from your search mentioned above.

Question 3 - There are many ways to get a 555 working and different people will recommend different arrangements. If you are looking for a stable, dependable circuit, stick the cap in there. Also, you can add a 47 mfd electrolytic from pin 8 to ground (put it close to the pin) for decoupling. I add these because they are easy to get and cheap. I do not add any superflous parts in my circuits - I believe in simplicity.

wow, gene thank you so much for your time!

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