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photo diode circuit modifications

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Full Member level 6
Jan 17, 2008
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Central Florida
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I attached a functional perspective of a circuit that I have. The basis of the circuit is that a red LED (part # LN1261CALTR) blinks based on rs-232 signals that are fed to it. There is a photo diode (part # SFH 2701) that is in close proximity of the red led that generates photo current in relation to red LED blinking. Since the photo diodes generates photo current and not voltage I have a transimpedance amplifier (part # OPA380) that converts the photo current to voltage.

So that is what I have.

What I would like to do is take the red LED and the photo diode out of the circuit. In other words just have serial data going to the transimpedanc amplifiers. But Im not sure if the transimpedance amplifier can handle voltage.

So my question are.

1) Is it possible to input rs-232 signals (voltages) into the transimpedance amplifier?

2) What if I convert rs-232 to TTL via a maxim 232 chip will the transimpedance amplifier then be able to handle the input as voltage instead of photo current?

3) If voltage input to the transimpedance amplifier is not the answer how else can i get a current into the amplifier without using the photodiode? Can a optoisolator work, if so please provide design details.

I attached block diagrams of my design as well as data sheets of my current circuit design and the circuit design I want to make.


  • 4n25.pdf
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  • LN1261CAL GW.pdf
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  • opa380.pdf
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  • what i have.jpg
    what i have.jpg
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  • what i want to do.jpg
    what i want to do.jpg
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  • FH2701_Pb_free_2008_12_04-16010.pdf
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A resistor can do the intended conversion.

Is it as simple as a series resistor from the DB-9 TX pin to the input pin of the OPA380 (transimpedance amp)?

The amps data sheet calls out 10mA max current. I measured the serial data voltage as 10V. A Resistor value of 5K provides 2mA current.

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