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Need a way to measure ohms

Discussion in 'Hobby Electronics' started by PèéJäy, Dec 5, 2003.

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  1. PèéJäy

    PèéJäy Guest

    Hi,

    I'm building a circuit where I need to connect a sensor to a microcontroller
    (AVR MEGA128). The sensor has a full value range of 5-100 ohms. I'm assuming
    because of the low resistance building a voltage divider would waste too
    much power so I need something else. The best I've found so far is:

    http://www.circuitsonline.net/circuits/view/27

    Only problem is that I would need to use two AD converters to measure the
    voltage and it doesn't run off 5v.

    Any Ideas??


    Paul.
     
  2. Rod Speed

    Rod Speed Guest

    Dud assumption.
    One way to measure resistance is to apply a constant current
    and measure the voltage dropped in the variable resistance.
    Those just cause steam to pour out of the ears.
     
  3. Terry

    Terry Guest

    PèéJäy threw some tea leaves on the floor
    A constant current source and measure the voltage using one a-d within
    the Mega128 ?
     
  4. gcd

    gcd Guest

    Hi,
    the a-d on the atmel is 10 bit, what sort of resolution do you need, 1 ohm,
    0.1 ohm ?

    for example

    if you connect a 1k resistor in series with your sensor and connect this to
    5V then you will have a range of ~ 25mV to 0.45V to send to the A-D. The
    a-d has a resolution of 5mV giving about 1 ohm resolution. You just need to
    do some math inthe micro to convert back to a resistance

    You could put an opamp with a gain of say 2 or 3 or 5 in between this cct
    and the atmel if you wanted better resolution.

    +5
    |
    1k
    |
    -------A-D
    |
    sensor
    |
    GND

    Cheers
    Greg
     
  5. PèéJäy

    PèéJäy Guest

    Hmm, didn't realise I would get such good resolution with a high value
    resistor. I was thinking something like 100 ohms which is a waste of power
    and might damage the sensor. ~1 ohm is fine.

    Thanks!
     
  6. If the Mega has an AD, then use the voltage divider. If you are worried
    about power, use a spare port pin to supply power to the sensor. Switch the
    pin on when you are making a measurement.

    Alternativly, if you have no ADC, charge up a cap and measure how long it
    takes to discharge through the sensor.
     
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