I need a little electronics help

[NorthWind]

Let me state right off that while I have a few years under my belt, I'm really a newbie in the elctronics world.

That said, I'm working on a project and could use a little help. It's a fairly simple signal generator based on the 555 chip. The schematics I have offer six selectable frequencies, and I would like to understand how one arrives at the values for the capacitors that control them. If I wanted to add or change a frequency, how would I figure out what cap to use?

Math lesson for a newb, anyone? I'm trying to learn.

The schematic is below.

Thank you very much in advance.

 

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[Carl-NC]

NW,

You can look this circuit up on the Internet, do a search on "astable 555". The frequency of oscillation is f = 1.44/(R1 + 2*R2)*C1. Ferinstance, for the values given (R1 = 3.9k and R2 = 68k) the frequency for the 10n cap is:

f = 1.44/[(3900 + 2*68000)*0.00000001] = 1029.3 Hz

Keep in mind that resistors and caps have tolerances (up to 20%) so the actual f will vary.

- Carl

 

[NorthWind]

Carl,

Thanks very much for making it basic enough for me to understand. I appreciate your time.