# 555 Timer Question



## xrunner (Jul 29, 2011)

I got a LED flasher kit from All Electronincs the other day. It uses a simple circuit with a small pc board, but there is a difference between it and the classic astable wiring. referring to the schematic below, it has R1 going from pin 8/V+ to pins 6/2 (not to pin 7). R2 and the rest is the same. A guy even mentioned this fact on the All Electronics comment board but nobody answered him. Is anybody familiar with why the circuit is designed like that? I have not put the kit together, but I assume it does blink the LEDs.


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## T-Man (May 16, 2008)

It does go to pin 7 so does R2.  It is how I wire my boards.
I will check some books 8 to 6/2? Right.

I can try it on a breadboard. I have one set up with no r1 or r2 but r3 is from 3 to 6 1 k and it flashes. What are the values for r1 and r2?


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## xrunner (Jul 29, 2011)

T-Man said:


> It does go to pin 7 so does R2.


Yea, but this one doesn't, and it seems to work because nobody complained about it on the comment area. But the one guy noticed the difference and sure enough, that's how the pc board actually is. Is it some weird way of wiring it up?


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## xrunner (Jul 29, 2011)

Reference -



> This kit is easy to assemble. It uses a 555 but the circuit is not the usual astable design (with a resistor between pins 8 and 7 and a resistor between pins 7 and 6 & 2). Your design has a resistor between pins 8 and 6 & 2 and a resistor between pins 6 & 2 and 7.
> 
> Why is the design different?
> Can I use the regular formulas to calculate frequency and duty cycle?
> ...


http://www.allelectronics.com/make-a-store/item/LEDKIT/LED-FLASHING-KIT/1.html


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## T-Man (May 16, 2008)

It does NOT worK. Is it a 555? I used 10k on 8 and 100k on 2-6. How about the cap size. I have 10 uf. 

You can have them alternate flash, that is easy to do They both go from three, one to 8 one to 1 with resistors and they are properly aligned. That is why I use a breadboard I mess with the numbers.

Draw it out with paint something else must be different.


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## T-Man (May 16, 2008)

First 2,6,7,8 are connected together. 2-6 go to 7 with r2. If 8 goes to 2 before going to 7 then r1 is r2 plus the resistence of what goes from 8 to two. With my values it does not flash. It may work but I need all the values to try it.

To answer that guys question R1 is really r1 plus r2. If he wants to use the calculation because 8 goes to 7 through the 6 r2.

The price is reasonable.


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## xrunner (Jul 29, 2011)

T-Man said:


> It does NOT worK. Is it a 555? I used 10k on 8 and 100k on 2-6. How about the cap size. I have 10 uf.


Yes it is a 555 chip.

They don't provide a schematic, or label components as R1, R2, etc. But looking at the board the resistor going from p8 to p6/2 is 120k - this would be R1. The R going from p7 to p6/2 is 33k (R2). The cap is 4.7 uF. It definitely positively 100% is wired that way on the board. It's wired just like the schematic below except for R1, as I've explained. r1 goes from p8 to p6/2! 










As you could read on the comments - no one said it didn't work, just the one guy noticed the uncommon wiring. It's either a coincidence that it works in some fashion that is uncommon, or the person who designed the board is some smart guy is trying to be cute and show off an odd way to wire the circuit. I will try it tomorrow or the next day on a breadboard as it's apparently wired. Can you try the components on your end as I've shown?

I just found this - isn't it wired just like I've described?


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## T-Man (May 16, 2008)

The diagram does not have the 4 to 8 connection so I will try again.

One interesting fact about the Rollins design is that there are no limiting resistors on the LEDs. For 9 volts they should have 200 ohms when in series.

Did you every look at the spec sheet for the NE555? I didn't see anything there.


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## T-Man (May 16, 2008)

This chip is so popular and books have been written on it. I own two. If I find a description I will pass it along but nothing yet. With flashing the outcome is important not neccessarily how. I mock it up on a breadboard and play with the variables to my taste. The typical astable operation is easier since all the formulas are given.


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## T-Man (May 16, 2008)

To breathe some life in this thread.

You can flash without a resistor at, R1 on pin 8

One example, it is just that with the typical astable configuration you can calculate the the frequency or flashes per minute. Frequency is per second.

This page is a Doozie No pin 7 use at all. A HYSTERETIC OSCILLATOR it's all there if you can understand it, This one a frequenecy formula

The moral of the story is that there are many ways to blink an LED.


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## Super Sugar (Jan 30, 2012)

Hey Xrunner, what software are you using to draw the above schematic? Or are they from the kit info supplied from Mark Rollins?


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## T-Man (May 16, 2008)

Both are links.


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## xrunner (Jul 29, 2011)

T-Man said:


> Both are links.


Right. I still haven't put it together, I'm trying to get my track wired up. I wanted to know if the standard timing calculation was valid for that circuit.


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## T-Man (May 16, 2008)

Something is missing. It did not work for me on the breadboard. How about a picture of the circuit board? For the price I think you can afford to assemble it. A breadboard will cost twice as much. Explain it? not really, if it works how does it look? No frequency was given.

This is a reason I stalk up. if I buy 1 555 timer to try it then I bought 50 later. Bulk is the way to go. Try something, if it burns, try another. The way to enjoy the electronics is to have the parts on hand. In the past I have preached Radio Shack which is convienent. Now I am learning more about selection. For example Radio Shack has a voltage regulator for 5 volts in a t 130 case. It works but I can get a smaller one to fit and work just as well from another source. I find it interesting, just to let you in on my philosphy.


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## gunrunnerjohn (Nov 10, 2010)

I'm a big fan of having the parts on-hand, so I buy in bulk from some of my favorite suppliers, much cheaper than places like Radio Shack!


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