OP-AMP question

[exapod]

In a motor controller circuit i found this op-amp and i have some doubts on how it works.
The IN of the op-amp is from a phototransistor used with an infrared led to count the number of holes on an encoder disk.
From i what i have understand the op-amp takes the non digital phototransitor output and converts into a square wave.
I'm only at first year of university and i have only see the ideal op-amp, so V+ = V- and the current that enter the two input is 0A.
Using this two things and the fact that the two resistor in the output forms a voltage dieider i know that V- = V+ = Vout * 10K (10K + 100 K) so Vout = (more or less) 10 Vin.
What is the purpose of the 1k on the + terminal? it looks like a pull-down resistor.
Also why there is a 10 k in series with the output? To limit the current that goes in the uC?
Thank you all for the help.

[Andy Watson]

Using this two things and the fact that the two resistor in the output forms a voltage dieider i know that V- = V+ = Vout * 10K (10K + 100 K) so Vout = (more or less) 10 Vin.

11 Vin, would be closer.

What is the purpose of the 1k on the + terminal? it looks like a pull-down resistor.

It probably is. You would need to know more about the opto-device - does the other end of the detector return to the positive supply?

Also why there is a 10 k in series with the output? To limit the current that goes in the uC?

Yes, probably. There could be several reasons for including the resistor, but limiting the current would be most likely. Does the op-amp operate from the same power supply as the µC? Don't overlook the possibility that the designer might have blindly copied the circuit from another piece of working kit - the resistor might not serve any function in this design!

[exapod]

The uC has the same power supply.
The opto-device is http://www.ottawarobotics.org/datasheets/CNB1302.pdf and it is powered from the same source of the uC and op-amp (3.3v).

[jeroen74]

The collector is likely connected to 3v3 and the emitter to the pull down resistor and the opamp input.

A current will flow when light hits the phototransistor, and the current causes a voltage drop over the resistor which is amplified by the opamp.

With a If (LED current) of 12mA about 600uA can flow, so the voltage over the resistor is 600mV; instead of the opamp a 10K load resistor probably would have worked too. A Schmitt-trigger would have been better.

edit:

I just studied the datasheet a little closer and I see the current transfer ratio can vary wildly, from 90uA to 880uA for a If of 10mA. So after all, amplifying it 11 times is not that bad an idea after all (I still would use an Schmitt-trigger)

[exapod]

Thanks for the answer.
If i use this https://www.sparkfun.com/datasheets/Robotics/QR_QRE1113.GR.pdf instead of the one used in the schematic can i use the same op-amp circuit? Is better to use a Schmitt-trigger? Can you point me one (Schmitt-trigger) that will work with this sensor?