How to calibrate the LEDs to zero Voltage.

Discussion in 'General Electronics Chat' started by Khalid Khattak, Aug 27, 2011.

  1. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    Hi,
    This is my first post in this great forum and i need your assistance. Before pointing out my problem, i will just say that i am very newbie to electronics. Kindly don't offend on my silly questions:
    Attached picture is a part of the Sun Tracker sensor based on Red 5mm crystal clear LED. Each LED generate 1.48 VDC when exposed to bright sunlight. The circuit designer has told me to calibrate the sensor with following procedure:

    Make both LEDs under bright light and then use the trimpot to get the output voltage zero. I have tried the method, but their is no change on the output voltage with full rotation of 5K pot. I have further explain the circuit in the attached file.

    Please help me. Whats i am doing wrong?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Reverse either D3 or D4. Then you will be able to zero the reading when both are exposed to bright light.
     
  3. Hi-Z

    Member

    Jul 31, 2011
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    That's a very unusual application for LEDs! Not that it shouldn't work, of course, and my guess is that you've connected the trimpot wrongly. My advice would be to re-check your wiring...
     
  4. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    SgtWookie,
    Thank you for the reply. If i reverse the LEDs then if i put a shadow on suppose D3 and the LED D4 exposed to bright sunlight will i get the voltage on DVM?
    Will i get the -Ve voltage if i reverse the above process?

    Attached is the complete circuit schematic of Sun Tracker. This is just to give you the complete pictures whats happening.
     
  5. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    Hi Sir,
    Basically these LEDs output voltages which then control the TL082P OP-AMP. If you kindly see the pictures i posted above especially the PCB one, can you point out how the Trimpot to be installed? The POT wiper is output of the sensor and this output is the input to OP-AMP.

    Regards
     
  6. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    To make it more clear, Attached please find the EAGLE PCB files. The eagle schematic is commented for the calibration of the SENSOR.
    I am thankful for the replies of the valuable forum members.
     
  7. Hi-Z

    Member

    Jul 31, 2011
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    Well, looking at the board layout, the trimpots seem to be connected in a sensible way - however the circuit diagrams refer to the wiper as pin 3, which seems slightly unusual. I would have expected pin 2 to be wiper, and note that if this is actually the case, then this would explain the unexpected behaviour.

    So, I think the next step is to identify the trimpot pinout, using a multimeter. We need to make sure the wiper is where you think it is.
     
    Last edited: Aug 27, 2011
  8. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    Thank You for helping me in this. I am attaching the Board file where i have shown the POT i am using. Physically the center pin is the wiper (which if you count from left or right is pin 2). However i have etched the same PCB, now question is by looking the P3 and P4 are you guessing the wiper pins are wrong?
    I will further check the wiper with Multimeter.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I was rather tired when I suggested to reverse one of the diodes last night, so forget that. I simulated it using LTSpice and a couple of voltage sources; it does work if you use t

    The author of the schematic created a custom pot, PT-05-S in discrete.lbr; seems that they simply used a non-standard pin numbering.

    I don't know why they created a custom pot, as CA6V (TRIM_EU-) in pot.lbr has the same footprint; it's just using oval pads instead of octagonal.

    If you are using SP1 for the 0v reference, you should be able to zero the pots.
     
  10. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    I will love to see the LTSPICE you created for this circuit. I still unable to zero the voltage. It always give me +1.48DC.. even i removed the pots and combined the LEDs output, i still get +1.48VDC.. I think the -1.48 dc and +1.48DC can't be cancelled out to zero, this is because one LED act as diode too...
    I will love if you can give me the value of POT i should use to calibrate the sensor?

    PS:
    i do use SP1 for zero volt reference.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    It's very simple. V1 and the two resistors really are not necessary to simulate it; it should just be a reading between N002 (which is your common terminal SP1) and the pot wipers.

    In the simulation, I'm using a 5k pot with the wiper centered.

    The blue trace is the voltage at the wiper.
     
  12. Khalid Khattak

    Thread Starter New Member

    Aug 2, 2011
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    Thanks for posting the LTSpice test experiment... I wish i could have a knowledge like you have...
    The culprit was the bad installation of LED.. This not means that i have installed the LEDs wrong way. The attached picture will clear what was happening. Actually the leds legs having notches that can not go inside the drill hole so i soldered the LEDs keeping the notches above the PCB. This made the LEDs 3mm above the PCB. The LED was disoriented i.e. The LED base was not square to the PCB and tilted. Hence, although i aligned perfectly the LEDS with the sun i was getting +1.48 volts.(because the LEDs face towards the sun were not centered )

    Actually what i did was to remove all LEDs .. Removed the long length of legs removed the notches.. and made the Led bottom flush with the PCB.. This way all LEDs lens direction become same and the LEDs are square to the PCB.. Now using artificial 1watt LED lamp i found that i can get ZERO voltage when the LEDS get equal exposure of light...
     
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