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Line following sensor circuit

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MikeK:
I've been experimenting with a white line following sensor circuit using an IR LED and IR phototransistor.

I see that most circuits online are using a 10K resistor for the phototransistor, but I've found this to give really awful response.

A 2K resistor works really well (at the emitter) and gives a full low/high swing when passed over a piece of white Styrofoam.  Any resistor larger than this doesn't work too well.

I don't think the IR parts I'm using are out of the ordinary, so I'm curious about the obvious difference in performance.

QED223 emitter: http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/QED223/?qs=sGAEpiMZZMvAL21a%2fDhxMh7mfA56bpVlGncOq5KiVY4%3d

QSD124 detector: http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/QSD124/?qs=sGAEpiMZZMs50KUSuyRkpqmW%252bYzaTd1qpijOcPsCWn8%3d

MikeK:
I should add that the phototransistor is powered from 5V and I have the resistor (and the output) at the emitter.

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