Re: 555 timer question: how to calculate required resistors and capacitor to achieve desired frequency?

redbelly wrote:
Ross Herbert wrote:
On 27 Nov 2006 17:52:17 -0800, "redbelly" <redbelly98@xxxxxxxxx>
wrote:


Ross Herbert wrote:

Using the 555 tool at
https://wwws.ee.ucl.ac.uk/facilities/teachlab/inter

Uh, that page seems to give erroneous results. According to the Texas
Instruments' 555 data***,

T1 (or T_high) = 0.693 * (Ra+Rb) * C

The ucl.ac.uk website has dropped the "Rb" term in their equation.

Mark


You're right, I didn't pay much attention to the text alongside the
circuit. I know it has been like that for years so I thought it would
have all the bugs out of it by now.

Tony Van Roon's 555 tutorial
http://www.uoguelph.ca/~antoon/gadgets/555/555.html looks to be a
pretty comprehensive teaching aid so maybe the OP can get more info
from it.

Funny how tough it is sometimes to be 100% error-free, whether it's
program code or just writing some text. It's mostly luck that I
bothered to check at all, but somehow the reference to a "50:50 duty
cycle" in your earlier post rang a warning bell in my head, because I
vaguely remembered that this was impossible with nonzero resistor
values.

Regards,

Mark

Yes, you can't get 50% duty with that configuration, but it can be
close. Ra needs to be a small fraction of Rb for near 50% DC which
means most of the resistance is in Rb. The values can be closely
approximated from Rb=0.693 / FC or C=0.693 / FRb and then just use 5 or
10% of Rb for Ra. But the error is big enough that .693 can be rounded
to 0.7. Also, Ra should be 1K or larger.

-Bill

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