Help with Flicker Circuit

[Tronceon]

Hello all;

I just edit the original post for this new one; however I've ran into a snag getting a 558 timer to run in astable mode. I've come across the AN171 Phillips "nxp.com/acrobat_download/applicationnotes/AN171.pdf" and I'm having trouble with the schematic they give me for such a process. Is the R1 and R2 the resistance already built into the chips Vcc or does it need to be external?

Thanks in advance,

Tron

 

[poor mystic]

Hi Tronceon
The webpage is not at the end of that link any more (or not now, anyway). However perhaps this is what you need to know:
The 558 is a quad 555 - there are 4 555's on the one chip. In the standard 555 there are 3 resistors, each 5K, which are used in a series potential divider at the input to a pair of comparators within the 555 chip. The output of these comparators drives the logic which in turn drives the power transistor used to drain charge from the timing RC circuit and provide the output signal. (All from ancient memory).
In case of further questions or doubts, it would help if you include an image of the circuit in question.

 

[(*steve*)]

Thanks for providing Troneon with an answer Poor Mystic.

Here is where that application note can be found now (I googled for AN171).

I presume he was talking about the test board layout? But as you have pointed out, there are the internal resistors for setting the trigger points and the external resistors used for setting charge/discharge rates.

Such information is more easily obtained from the datasheet.

Looking at that you will find that Vr for the 558 is set by 2 internal resistors (marked R1 and R2). So, yes, these are internal.

The 558 consists of 4 timers that are not 555s -- it is not like the 556 (which contains 2 555s). These timers are perhaps "inspired" by the 555.

Also not that the datasheet I references above incorrectly shows pins 7 and 8 to be "output". Pin 7 should be "Timing B" and pin 8 should be "Output B".

Having said all of this, the original post was from almost exactly 2 years ago, and I would be hopeful that Tronceon has already found his answer. I trust he doesn't mind getting another -- since he may get email notice of your reply Poor Mystic

 

[poor mystic]

yeee-eesss. hmmm. thanks.