[sdiy] Impedance / equivalent resistance calculation

[Bert Schiettecatte]

Hi,

> your DC impedance at IRIN1 is parallel connection of IR5 and 
> IR9, that's 750ohms. If you expect to detect the light at 
> frequency highier than, say, 40Hz (twice corner frequency of 
> filter made of IR5,9, IC9), cou could consider the cap as 
> short. Your node with IR1 and IQ1 has equivalent resistance 
> of 690 to 2.1k (again, parallel of IQ1 and IR1 in light and 
> dark), so overal AC impedance is more or less (690-2100)||750 
> = 360 - 550 ohms

That's what I thought too but I was not sure I could look at 
The 10uF cap as a 'short'. 

Now, I do understand that IC9 and IR9 are forming a high-pass 
Filter, and that IC1 and the equivalent resistance before it 
Will form a low-pass filter. Are there any other filters I am 
Missing here? I don't see how IR5 is involved in a filter.

Thanks,
Bert.