This seemed worth coming back to since it was buried at the bottom of my "Using the 555 timer as an external clock for the Arduino" article, but I have built a calculator for oscillator timing with the 555.

I know, I know, there are a lot of those out there. Well, that's half true. There are a lot of calculators that will take the resistor & capacitor values and tell you what the resultant timing would be... but usually you start from the other direction... you have a timing spec in mind and you need to know what components to use. That's how I tackled it.

You tell it that you want the timer to fire every 60 seconds, or at a frequency of 440Hz, and it will make recommendations about some common components that will have that effect, along with duty cycle, etc.

Because my blog software doesn't like adding javascript very much, I put the calculator on it's own separate page: http://totusterra.com/555timercalc.html

Hopefully this will be useful to anyone playing with the 555, with or without the Arduino involved.