[sdiy] Impedance / equivalent resistance calculation

[Bert Schiettecatte]

Hi All!

I have another (probably silly) question about this circuit:

http://www-ccrma.stanford.edu/~bschiett/impedance.png

I need to find out what the impedance at the IRIN1 net is, since 
It will be connected to an A/D pin. I suppose I need to calculate 
The equivalent series resistance of this circuit. 

I measured the phototransistor to be 75K under dark conditions and 
1K under light conditions. 

What is the correct way to calculate the equivalent resistance? 

I calculated the eq. resistance for the transistor and IR5:

Rd = RDark * IR5 / (RDark + IR5) 

And

Rl = Rlight * IR5 / (Rlight + IR5)

And then for IR1 and IR9:

Re = IR1 * IR9 / (IR1 + IR9)

But then... There is a capacitor in there and that's where my 
Knowledge of calculating equivalent resistances ends :-)

Thanks in advance,

Bert