Problem with PIC18 ADC port

Discussion in 'General Electronics Chat' started by arthurprs, Dec 12, 2011.

  1. arthurprs

    Thread Starter New Member

    Apr 16, 2011
    Hello everyone

    I'm testing the following circuit for a line follower robot.


    I have set up the PIC to power a LED on port C0 when the voltage over the 100k resistor goes above 3.91V (collected with port A0)

    For some reason the LED is always ON, with or without a white surface in front of the sensor!

    When I connect my multimeter to check the voltage over the 100K resistor, my LED turns off as previously expected... I can confirm the reason with the multimeter, the voltage is ~2V.
    With the multimeter connected it works as expected. If I put a white surface in front of the emitter and photo-diode the uC lights the LED in C0.

    What I'm missing? Thanks.
  2. arthurprs

    Thread Starter New Member

    Apr 16, 2011
    I definitely need better tools, my multimeter set to read up to 5Vdc have only 20K internal impedance! So my 100k resistor was way higher than I needed (I calculated its value using the multimeter...). I replaced the 100K with a 2x10K in series and it worked fine.
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    You can get a decent DMM from harbor freight for $3.

    They aren't the most perfect, but have high input impedance and acceptable accuracy for these tasks.

    If you plan on being in the hobby for a long time, I'd suggest a Fluke 87-III or 87V from eBay.

    There are many meters in between with varying accuracy and price.