Wheatstone Bridge

Discussion in 'General Electronics Chat' started by RoyalDeviLz, Jun 24, 2014.

  1. RoyalDeviLz

    Thread Starter New Member

    Jun 24, 2014
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    I'm currently trying to build a Graphite sensor using only paper and 2B pencil. I attach my paper graphite sensitivity to one of the resistance connection of my wheatstone bridge circuit along with 4 others 100k resistors. I able to achieve a minor changes in output voltage of 0-1V when the resistance of my graphite tends to go high, but i wan to amplify the output signal to a higher extend in order for my microcontroller to detect it more effectively. I used so call instruementation amplifier configuration with 3 LM741 amplifier. But the output voltage i constantly get is within 4.010V to 4.015V only. The changes is too minor for my microcontroller to detect the analogue waveform. Any expert here have any idea on which kind of amplifier I should implement in this case?

    My circuit design is show in the attachment...

    I wan make it like 0.1V changes in the bridge circuit, I will get approx 0.5V changes at the Amplifier stage output..
     
    Last edited by a moderator: Jun 24, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi RD,
    A quick look at your circuit shows that with a 100K gain Pot you have an attenuator not a high gain amplifier.
    Try a 1K pot in place of the 100K gain pot.

    Also with 500k 'graphite' resistor the bridge will be greatly off balance.

    The 741 OPA is a poor choice of amplifier, especially with single voltage supply.

    E
     
  3. RoyalDeviLz

    Thread Starter New Member

    Jun 24, 2014
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    For the 100k and 500k pot i can adjust my resistance value to a lower state, but the output from the amplifier stage wasnt what i demanded.. I saw many using 741 opamp as amplifier circuit, so juz wanna try with this..or you have any other solution for me?

    My graphite resistance could varies from 100-200k ohm, i just wanna change this resistance value to a voltage signal form, so i can convert it using ADC and simulate it using Matlab.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    To adding a voltage comparator using 741 on the end of the 741-3, take R12 away, and adjust the reference voltage to match the sensitivity you want.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    With a bridge built using 100K resistors, if one resistor [ the graphite ] changes to 100080 ohms [ a 80 ohm change] the bridge ouput will be 1mVolt.
    800 ohm change 10mV
    8300 ohms 100mV.

    Using the last value of 100mV, the amplifier will require a gain of 50 to give a 5V output.

    The problem is the 741, if powered by a single +5V supply will never give an output higher than approx +3.5V, also the output will never go lower than ~ +0.8V.

    The 741 is not suitable for this application.

    E
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,744
    Look for a rail-to-rail output op amp. The 741 output is typically limited to 1.5 to 2 volts above ground (as a minimum) and, on the high side, limited to 1.5 to 2 volts below your power supply (3 to 3.5 volts if using a 5 volt supply).

    As you see, an op amp with a working output voltage from 1.5 to 3.5 volts (possibly 2 to 3 volts) will not be very useful. His device have additional limits on input voltages as well.

    Look for an op amp with rail to rail output. Or, at least, 0 to 4.2 is a common configuration for some TI op amps when powered with 5 volts.

    Cheers
     
  7. RoyalDeviLz

    Thread Starter New Member

    Jun 24, 2014
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    ok, what are the amplifier are suggested to use in this application to achieve higher and better sensitivity from resistance varies? or any configuration of circuit you can suggest to me? I really need help on this.. thanks..
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    Hi,
    I use the MCP6002 dual rail to rail OPA, they are not expensive.

    Have you done any test work in measuring the resistance of the graphite 'track' on paper.??

    It should be possible, using a DVM on the resistance setting, to measure the resistance values of different track lengths and widths.
    You should also try to measure highest resolution by moving pencil/probe on the graphite track
    The results you get will determine what level of amplification you require, lets know what you measure.

    E
     
  9. RoyalDeviLz

    Thread Starter New Member

    Jun 24, 2014
    18
    0
    Ok, i will try using that amplifier. I'm actually trying to build a graphite sensor using only paper and pencil to determine the vibration. I sketched 5cm square surface area of graphite on a sheet of A4 paper and measured with a value of roughly 100Kohm +-, the value was very unstable actually. So i would try to use a signal conditioning circuit to determine the analogue signal from the sensor.
     
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