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Darlington pair setup for QRD1114 IR phototransistor circuit?

Discussion in 'General Electronics Discussion' started by rbatty91, Nov 20, 2013.

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  1. rbatty91


    Nov 18, 2013
    Hi guys,

    I'm building an optical tachometer (as so many people have done) and using a QRD1114 sensor (datasheet: to calculate the rpm of a toothed pulley wheel (28 teeth). I've sanded the wheel to make the teeth nice and shiny and filled the troughs with black marker to give a big difference in reflected light.

    I've set my circuit up as shown here for the sensor: (scroll half way down the page)

    When I do an analogue read on the pin attached to my sensor, the voltage isn't very well defined i.e . high's of 3.4V and lows of 0.8V. I know I can change my resistor values for better sensitivity however I want to implement a darlington pair to give a square wave with a nice transition from 0 to 5V on every 1/28 rotation.

    How do I go about setting my circuit up? I have been looking at this diagram for a photodiode: however I am struggling to replace the photodiode from the circuit shown here with my QRD1114 sensor. Can I do it this way?
  2. BobK


    Jan 5, 2010
    Use a Schmitt trigger to clean up the signal to a square wave.

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