simple oscillator formula

[soadrage7654]

Hello, I am working on a simple oscillator that uses two inverter gates, a resistor and a capacitor. I planned this out a while back, but I cannot find the formula I used to calculate the frequency. I also can't find anything online that resembles the circuit that I am using and also gives a formula for it. Can someone help me out here? I attached the schematic.

Thanks,
Josh Clark

 

[Sceadwian]

**broken link removed**

You may want to add one more inverter as a buffer to the LED separately from the feedback or the LED loading will effect the oscillator frequency, how much I don't know.

 

[dougy83]

I would assume that the following would be correct:

F = x/(RC)

Just build the circuit and measure the frequency. Then you can calculate x. Then you can calculate the rough frequency for any RC.

 

[soadrage7654]

Ok thanks for the suggestion. I was going to use that as a last resort, but I guess it's the only choice now.

I just thought it was weird that I had a formula for the frequency when I first planned it out, but I couldn't find it later so I wanted to check to see if anyone else happened to know it.

 

[dougy83]

Actually, before you give up, have a look at
**broken link removed**

You will note that there is an extra resistor used, Rp, to 'protect' the CMOS input. It also has the added function of preventing the capacitor from being discharged through the CMOS input diodes (this affects the duty cycle & frequency, so the above f=x/r/c is no good due to the diodes affecting the timing)

 

[Hero999]

Rp is just a high value resistor 100k to 1M is normally good enough.

 

[MrAl]

Hi,

Someone else asked about this and we talked about it can came up with
very good formulas. If someone could find it, it is here on this site somewhere.