Obtaining Collector Current Vs lumens for photo transistor setup

Discussion in 'Embedded Systems and Microcontrollers' started by guitardenver, Nov 14, 2014.

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  1. guitardenver

    Thread Starter Active Member

    Jan 24, 2009
    If I have a photo-transistor, specifically this one WPT-1CL3H (Yes, it must be that one), I would like to obtain a graph of The collector current vs how much light the transistor is picking up.

    The setup:
    The emitter of that transistor is pulled down to ground with a 1k resistor and the collector is connected to +3.3V. Then a microcontrollers ADC is connected to the emitter to read the voltage across said resistor.

    I need to vary the light going into the photo-transistor with an LED (WP710A10MBD) that is controlled by the MCU. I tried to use PWM to vary the light intensity but the photo-transistor responds to fast and the ADC values are all over the place.

    Does anybody have any creative ideas of how I can vary the light with a continuous light intensity? Without the use of other physical objects like paper with different thickness/color to vary the light (That is how I do it already). What are the implications of doing PWM at a speed in which the LED nor the photo transistor can respond to fast enough? Make it more "analog". The MCU speed is limited to 4MHz. Also the LED is not on a hardware timer output on the MCU, so the pin state that controls the LED must be toggled in software. I'm dealing with old unchangeable hardware, which means I can not add a variable current source circuit or something like that.

    Thanks for the brainstorm.
  2. Wendy


    Mar 24, 2008
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