Op Amp Scale Shifter

Discussion in 'The Projects Forum' started by hoohbener, Jun 9, 2013.

  1. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Hi all,
    I am a new member and I am currently working on an analog self-stabilizing platform. I need some advice about interfacing the accelerometer sensor.

    For this project, I will use a 555 timer circuit as the processor which will modulate pulse train -based on the control voltage input pin 5- for the actuator (I'm using a servo, GWS S03T).

    The sensor I am using is a MMA7361 modul from DFRobot, it outputs analog voltage in linear manner from 0,85V @-1g to 2.45V @+1g on each axis.

    The problem I encounter is that the MMA modul works on 3.3V and the outputs are inside the scale of third of this supply voltage rate (.85 to 2.45), while the input pin 5 of the 555 timer works inside the third of the minimum 5 volts operating voltage of the timer IC. so I need to shift the MMA output.

    Anyone have suggestion/s?

    I've tried to use Op Amp to amplify the outputs but then with any gain the "center voltage rate" is still differ from the pin 5 requirement.

    Thank's for any help and sorry for my bad english.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    You can shift the voltage output of the opamp by using it as a summing amp.
    You do this by applying a reference voltage into either the inverting or non-inverting input through a series resistor.
    This is one of the basic opamp configurations that you will find in any opamp textbook.
     
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  3. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Hi there MrChips,
    Using summing amp will create negative output isn't it?
    I've been simulating (using Falstad' Circuit Simulator Applet) the "voltage difference amp" from the link below
    http://www.societyofrobots.com/schematics_voltamp.shtml
    and playing with the resistors value.
    Attached is my screen shot on the simulation.

    The two op amps is actually one, i'm using two to represent the low and high limit output from the MMA modul.
    This arrangement gives out 1.52V to 3.53V inverted output which is close enough to the expected 1.67 - 3.3 V.

    I think this might work because with the output level, the op amp will not be saturated. But learning from the previous experiment, the op amp is quite an unstable one, probably the one I;m using, the uA741.

    Do you think this might work?

    because the reference schematic suggests to use same value of resistors (R1 and R2) on each half of the voltage divider of the non-inverting and inverting input.
     
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  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    Your basic circuit would be something like this. This is just some random example from the net. In your case I think you can use some simple single supply opamp like LM324. What is important here is to stay inside the opmap input common mode specs. NOTE Also Select resistors in a range of 10Kohms to a few 100Kohms
    [​IMG]
     
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  5. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Thank's T06afre.. can't find your real name on your profile, but did find that we have the same birthday date, anyway..

    Could you explain more about why we should use high value resistors?
    Is it the op amp requirement related to the "no current at inputs" rule?

    Could you post the link of the example above?

    I'm new in electronic, this is actually my first month of learning (in order to build my own MoVI stabilizer), so, thank's for any helps.
     
  6. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Okay, I got it T06afre...

    After some close observation to the simulation, I finally noticed that the current flowing into the output of the MMA modul if low value resistors are used.

    Correct me if I'm wrong, we need the high value resistors to prevent damage to the MMA modul caused by the negative current.
     
  7. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Here's some update, so I thought it's the negative current that makes T06afre suggestion.
    I've added two pull down resistor to divert the current to ground (sim' schematic with 555 timer connected is attached).

    I really hope to find some approval or correction from the other member, it would be much appreciated!!
     
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  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    Your circuit is still somewhat over complicated. Why use two opamps?
     
  9. hoohbener

    Thread Starter New Member

    Jun 9, 2013
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    Actually, the two op amps in the simulation is just my way to see both the low and high limit (the voltage swing) from my sensor's output and what voltage they will produce after passing through the op amp. In the physical circuit, I will use only one op amp.

    Anyway, I tried this on my breadboard and didn't get what the simulation showed.
    So op amp is out.

    What I'm trying to do now is to make a simple voltage divider with the 5 Volt Vcc on one end and the sensor's output voltage instead of ground on the other end. Both ends are pulled down with resistors to eliminate the negative current. Schematic attached.

    I don't know if this method (voltage divider from two positive voltage sources) is commonly used, so any suggestion will be much appreciated.

    It's just too soon for me to move into Microcontroller.
     
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  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
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    Whats wrong with using a single transistor and a couple of resistors?

    (Double that if you need a non-inverting amp)
     
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