cheap voltmeter

Discussion in 'The Projects Forum' started by Tom88, Jan 6, 2012.

  1. Tom88

    Thread Starter New Member

    Oct 14, 2011
    4
    0
    Hi guys im trying to build a voltmeter. Figured using the 7129 chip would be easiest, since it has everything included internally. Ive seen some diagrams how to implement this but still a little confusing. Mostly the amplification stage. I came up with a little schematic on how i think it works, let me know if right or wrong here, but does it use a 3p4p switch to control between the voltage drop on the resistor chain and the correct output voltage? Because it seems it should be easier than that.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Unless you need to measure below the full scale sensitivity of 200mV for the 7129 you don't need to add the amplifier. You just need the resistive voltage divider attenuator.
     
  3. Tom88

    Thread Starter New Member

    Oct 14, 2011
    4
    0
    2000mv full scale. How would I implement that, when the Vin is different from the output of the divider. There needs to be some amplification
     
  4. PaulEE

    Member

    Dec 23, 2011
    423
    32
    If your input voltage is +5v and you want to measure that on the 200 mV range, you can use a divider to divide the +5v down to a voltage between zero and 200 mV.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    For 200mV sensitivity you just connect the ADC input directly to the top of the attenuator that goes to to the voltmeter input terminals (the attenuator divider typically has 10megΩ total resistance to ground). For safety you should connect a resistor in series with the ADC input with two inverse parallel connected diodes to ground.

    Edit: Here's an app to help calculate the values of a resistive divider attenuator.
     
    Last edited: Jan 6, 2012
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