Signal conditioner with constant mid point

Discussion in 'The Projects Forum' started by wstuck, Jun 16, 2009.

  1. wstuck

    Thread Starter Member

    Jun 16, 2009
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    Adjustable signal conditioner with constant mid point.:confused:

    I need help with a joystick project I am working on for disabled people.:)

    The input device is a 2 axis joystick with a 5V DC supply and a 2.5V midpoint in 2 axis. This is an electronic device and not two pots. The input voltage must stay at the 5V to make the joystick function.

    The Controller works in a closed loop mode comparing the input signal and the feedback signal from two linier 10K pots (1 per axis)

    The midpoint for both input and feedback is 2.5V with a range of 0V min - 5V max

    The controller works fine as it is setup but I would like to be able to vary the input signal from the current 0V to 5V (2.5V midpoint) to some of the following by using a pot and op-amp or some circuit that will keep the midpoint at 2.5V.

    .5V to 4.5V (2.5V midpoint)
    1V to 4V (2.5V midpoint)
    2V to 3V (2.5V midpoint)

    I have a link to the basic design layout at http://www.designabilityinc.com/help/Control_Conditioner_Rev_A.ppt


    Thanks for any help or suggestions.
    www.designabilityinc.com
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    Do you want a selectable sensitivity?
    Is the input always 0 - 5 Volts?
    An opamp circuits with a selectable gain would work.

    Greetings,
    Bertus
     
  3. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    The input is always 0-5V but the midpoint needs to be 2.5v to work with the feedback pot and the controller. I would also be using a 12V battery for the power supply. I do not have a negative voltage for the op-amp.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Give this a shot.

    Not all capacitors are shown; the 78L05 should have a 0.1uF cap on it's input, and there should be another 0.1uF cap across the LM324's power pins. You could substitute a 7805 if you'd like, but in that case replace R1 with a 1k pot. Either way, R1 should be a 10-turn or more trimpot.

    Also, the LM324 is a quad opamp; the unused amps should have their noninverting (+) inputs wired to a voltage between Vcc and GND, and the output wired to the inverting (-) inputs. I chose this opamp because it's available even at Radio Shack, it's single supply, it can sense near ground, and Vout can go to ground.
     
  5. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    I have a few more questions if possible? The image is hard to read.

    Is this configuration set up as a non-inverting op-amp?
    Will the midpoint or null condition stay at 2.5V?

    Thanks for your help.
    See the project at www.designabilityinc.com
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're using Microsoft Internet Explorer, it re-sizes the image to fit inside the browser viewing window, which distorts the image.

    Click on the thumbnail to open the image, and then click on the image to toggle it between the compressed image and the full image.

    It's set up as an inverting opamp. U2 is a 5v regulator. R1 allows adjustment of the midpoint; it should be set to 2.5v.

    V2 represents the 5v supply you're feeding to the high side of the joystick; it's shown varying between 3v and 5v. The opamp compares the noninverting input to the inverting input, and basically mirrors what's happening to the 5v side, going from 2v to 0v instead. R4 represents your joystick.

    Note that I have R4 shown just a bit off center. If it were exactly 50%, it would overlap the green trace at 2.5v.

    All semiconductors drift over time, and particularly over temperature. You'll have to "tweak" things a bit now and then.

    If you want it to be more precise, you'll need to purchase a precision 2.5v reference. I just used commonly available stuff.
     
    Last edited: Jun 17, 2009
  7. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    Thank you for your help. I will buy some parts and see what happens. I was also able to print the image without any problems.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    Sorry for the late reply.
    Here is my version of the signal conditioner.

    [​IMG]

    The TL431 is a 2.5 Volts voltage reference.
    The remarks on the LM324 SgtWookie gave are also to be applied here.

    Greetings,
    Bertus
     
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