Low Voltage Comparator

Discussion in 'The Projects Forum' started by rh77abc, Aug 10, 2014.

  1. rh77abc

    Thread Starter New Member

    Aug 10, 2014
    This seems simple from what I've read, but having problems. Trying to use a 741 op amp to compare two voltages. I need to be able to compare voltages between 1 and 3 volts. These voltages are from a .5-4.5 volt sensor and a reference voltage from a potentiometer. I want to turn a buzzer or led on if the sensor voltage drops below a variable set point.

    I'm using a battery at 7.6 volts, I have also tried powering everything thru a 5 volt regulator. When I connect power to the 741 with nothing connected to the inputs, the output goes high at 6.6 volts. If I connect the inverting input to the positive 7.6 volts the output only drops to 1.2 volts; I expected lower but I guess not a big problem.

    The problem is if my sensor voltage at the inverting input drops below 1.9 volts, the output goes high regardless of the reference voltage at the non-inverting input.

    I have also tried a 1458 op amp with similar results.

    These two op amps were just what I had, but what I've read seem to indicate they would work. Do I just need a different op amp or a comparator?

    Any help with comparing voltages between 1 and 3 volts would be appreciated.
  2. wayneh


    Sep 9, 2010
    I recommend a genuine comparator such as LM339, which has 4 comparators in one package. It's purpose designed for your exact task and is widely available.

    The problem with your op-amps is that they cannot sense towards either power rail and cannot output close to the rails either. There are more suitable op-amps that can, but there's a reason they make genuine comparators.

    One note, the output of a typical comparator is either low (can sink current) or floating (cannot source current). So you usually use an external pull-up resistor to get the "high" signal. It's all described in the data sheet.

    The comparator can sink enough current to light an LED (~5mA) but you'll need an external transistor to drive a buzzer.
    rh77abc likes this.
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    Seems you would just need to connect led with negative leg of led connected to op amp output, positive leg of led connected to positive terminal of the voltage source. When op amp output goes low, there is voltage drop across the led, led turns ON.
  4. crutschow


    Mar 14, 2008
    The ancient 741 was designed for ±15V supplies and works poorly nor not at all with less than 10V across the supply pins. The input and output will also not go more than within about 1.5V of the supply rails. So the 741 is a bad choice for your requirements. Use a comparator such as the LM339 as wayneh recommended.
    rh77abc likes this.
  5. eetech00

    Active Member

    Jun 8, 2013
    Hi :)

    Here is window comparator circuit simulation that will light LED if voltage IS NOT between 1v and 3v using LM339 comparator. :rolleyes: But, I'd rather have it light if the voltage IS between 1v and 3v.

    rh77abc likes this.
  6. to3metalcan


    Jul 20, 2014
    Change U1: C to an inverting comparator (basically just switch what's connected to the +/- inputs), or use U1: D to invert the output (tie an equal-resistor bias voltage divider to the + input and connect your signal to the - input.)
    rh77abc likes this.