Adjustable signal voltage control with 2.5v center

Discussion in 'The Projects Forum' started by wstuck, Nov 28, 2011.

  1. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    Adjustable signal voltage control with 2.5v center

    I need help finding a kit or parts to solve what I thought would be a simple problem.

    I have a motor controller that has the ability to have analog input devices like a potentiometer or an analog joystick. The input signals are 0v-5v DC with a 2.5v center point. When the analog device is calibrated correctly it is easy to achieve the 0v-5v DC.

    But what if you want to add gain to the input signal and maintain the 2.5v center or add loss to the input signal and maintain the 2.5v center. Or what if you replace the analog device with a hall effect sensor that has voltages that are not quite 0v-5v DC? Another example is to have limitation adjustability so the center is always 2.5v but the min and max can be changed to something like 1.5v-3.5v with the same 2.5v center

    Thanks for any help or suggestions.
     
  2. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    I have attached a circuit that I built to do the following but I would like some help making it better.

    Steering signal has the ability to minimize the signal voltage and maintain the 2.5V center.
    Steering Example:
    If input is 4v I can limit it to 3v by turning the pot and still maintain the 2.5v at center.
    If input is .5v I can limit it to 2v by turning the pot and still maintain the 2.5v at center.

    Throttle Example:
    If I turn the switch from zener diode to zener diode I can clip the max input signal but still maintain the 2.5v at center.

    Making a better Circuit:
    Steering – I would like to be able to add gain to a week signal if needed on the steering input and add loss to a strong signal. I do not have any problem adding a switch if needed to change from loss or gain.

    Throttle – I would like to have a more variable zener diode replacement to make the voltage clipping point more adjustable.

    See attached image and Circuit Maker File.

    Thanks for any help.
     
  3. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    See attached Circuit Maker File.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I've extracted your schematic image from the Word document.

    It is best if you post attachments in .png format; as no other software to view it is required, the images are compact, and not "lossy" like .jpg is.

    The idea of clamping the output of an opamp is not the best. When the Zener starts to conduct, the opamp will still try to maintain the output voltage, and will output maximum current in its' effort to get the output voltage where it "thinks" it needs to be.

    You have the speed and steering joysticks connected so that they will interact with each other (undesirable) and the "2.5v ref" is high impedance, so it will vary all over the place.

    You are using 1458 opamps. Those are basically a dual 741 opamp. You should get rid of those before AudioGuru goes on a murder spree.

    Are the "ends" of your input potentiometers accessible? If so, it would be better to feed the ends of their pots from your circuit so that you could set an initial balance, and thereafter it will be maintained.

    You should review our E-book section about operational amplifiers:
    http://www.allaboutcircuits.com/vol_3/chpt_8/index.html
     
  5. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    Thank you for taking the time to review the circuit and for your advice. I will check out the reference material that you suggested.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It will help you quite a bit if you understand how opamps work, and how to apply divided and negative feedback (along with summer circuits, etc) to get the desired gain.

    When used without feedback, opamps have extremely high gain. You almost never want to use an opamp without feedback.

    If the output is connected to the inverting (-) input without a divider, you have a voltage follower. Both of the opamps in your circuit were connected as voltage followers. Yes, you had 10k resistors in the feedback path, but there was no other resistor to a reference point to create a voltage divider, and the inputs to voltage opamps are very high impedance - so you essentially wired the outputs directly to the (-) inputs. This is OK if you wanted to create a voltage follower/buffer, but that was not your intent.

    Once you start "getting the hang" of divided feedback, try experimenting with the circuits presented in the E-book.

    Instead of the 1458, consider using the LM124 or LM324 if you are using Circuitmaker Student, or an LM6132 if you are using Circuitmaker 2000.

    Also, you'll find your schematics easier to look at if you hit F5 (Schematic options) and un-check the "Show Pin Dots" box (or hit the "i" key to toggle check/unchecked).
     
  7. wstuck

    Thread Starter Member

    Jun 16, 2009
    14
    0
    Op Amp update:
    The op amp on the drawing is an MC1458 but the one I used in the real circuit is an ALD2702A 5V rail-to-rail.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Which Circuitmaker are you using? Student? 2000? A different version?

    I already gave you a couple of suggestions in my prior post:
     
Loading...