I'm working with this circuit to design an ambient light meter.(adsbygoogle = window.adsbygoogle || []).push({});

http://img201.imageshack.us/img201/6479/cktyu0.jpg [Broken]

We were given steps to go about solving for different values of the components in the circuit.

First we're to solve for the gain and thus V1 which I found to be

[tex]V_1=\frac{-R1}{(Rphoto+R7)}*Vee[/tex]

Then we're to solve for V2, which I found by NVA:

[tex] V_2 = \frac{R3*V1+R2*Vee}{(R3+R2)}[/tex]

I don't think there is anything wrong with either of those equations, as I've verified that they give the correct voltages when simulating the circuit in PSpice.

The given\known values in the circuit are Vee=-10V, R1=1k and the condition when Rphoto=400, V2 = 8V and when Rphoto=10k, V2=0V

This leads to:

[tex]8 = \frac{R3*\frac{-1000 \Omega}{(400 \Omega+R7)}*-10+R2*-10}{(R3+R2)}[/tex]

and

[tex]0 = \frac{R3*\frac{-1000 \Omega}{(10000 \Omega+R7)}*-10+R2*-10}{(R3+R2)}[/tex]

With those 2 equations, I am to deduce values for R2, R3, and R7, which I can't seem to do having only the 2 equations. Can anyone push me in the right direction? Thanks.

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# Homework Help: Circuit design, 2 equations, 3 unknowns

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