Variable dual comparator.

Discussion in 'The Projects Forum' started by Jamie N, Dec 13, 2008.

  1. Jamie N

    Thread Starter New Member

    Dec 13, 2008
    The problem is that I need to have a single analogue VDC output from a pair of VDC inputs. Both inputs are 'variable' +0 to 6VDC and with there returns to ground. I need to monitor the differential between inputs #1 and #2, where #2 will be either the same OR greater. This differential will be variable up to a maximum of +4VDC and will simply be read on a Voltmeter.
    Whilst not being a total buffoon, I'm looking for absolutely the simplest solution to this, which might be a 'comparator', but I can't find one which says 'THIS ONE' in large enough, or simple enough letters for me..!:confused:
    Any assistance with this frustrating issue would be truly appreciated.
  2. SgtWookie


    Jul 17, 2007
    You're looking for a Differential Amplifier.
    Please see our E-book, here:

    For simplicity, you may wish to use 10k Ohm resistors for all four in the top schematic. You could use greater values if you wish. The precision of the resistors is important; I suggest using metal film 1% tolerance or better.

    If you wish to run from a single supply voltage, you could use an LM324; connect all unused noninverting (+) inputs to ground, and all inverting (-) inputs to it's respective amplifier's output. You will need a voltage source at least 1.5v greater than the highest expected input or output. A 9v "transistor" battery should suffice.

    Use a 0.1uF (100nF) capacitor across the opamps' Vcc and GND pins to help keep things quiet.

    Note that the lower voltage of the two signals needs to go on the inverting (-) input.
    Last edited: Dec 13, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If you obtain an inexpensive digital panel meter which has a floating input, the display will indicate if the one voltage is negative relative to the other, plus read the magnitude of that difference. See the link for an illustration -

    One can also set up a pair of voltage comparators to indicate which voltage is positive/negative relative to the other, but that provides no measurement of the amount to difference voltage.
  4. Jamie N

    Thread Starter New Member

    Dec 13, 2008
    Thank you both very much for your responses. I think that you've saved me a great deal of time and hassle here.
    Have an extra beer from me and put it on my tab!