6V LED Voltage Monitor

Discussion in 'The Projects Forum' started by MBragg, Aug 13, 2012.

  1. MBragg

    Thread Starter New Member

    Aug 13, 2012
    I am interested in creating a Voltage monitor using an RGB LED. Currently I am using testo voltage monitors, but I need to soon test about 48 on a continuous basis, and I would like to have a quick visual indicator that the device is receiving the 6V supplied. Ideally the circuit would allow for 5.7V-6V Green, 3V-5.7V Blue, below 3V Red. Any ideas or alternate suggestions would be greatly appreciated. I do not want to use a siren due to the working conditions, just want a quick visual indicator if all 48 of the devices are receiving the 6V supply.
  2. bertus


    Apr 5, 2008

    How about green orange and red for colors?
    You could use two comparators and a bi-colored led.
    One comparator will let the green burn above 3 Volts.
    The second comparator will let the red burn below the 5.7 Volts.
    Between 3 and 5.7 both leds will burn and show the orange.

  3. mcasale

    Active Member

    Jul 18, 2011
    A simple set of comparators and some resistors to set thresholds should do it.

    You will probably have to use a transistor for the 10- to 20-mA LED current.

    The tolerances of your supply voltage and resistors will determine how tight the thresholds are.
  4. wayneh


    Sep 9, 2010
    Those cheap battery testers in the hardware store will give you a similar result for a 9v or 1.5V battery. I think with minor tweaks - probably just the resistors in a divider circuit - you could modify it for your 6V device. These things usually use an LM324N quad op amp, I think because it can drive an LED directly. See here and here, for instance.
  5. SgtWookie


    Jul 17, 2007
    An LM324 can sink up to around 6mA reliably. That's plenty for an indicator LED.
    Make certain that you get LEDs that have a wide viewing angle.
  6. MBragg

    Thread Starter New Member

    Aug 13, 2012
    I hate to ask, but do you know the values I would need to change in order monitor my values?

  7. wayneh


    Sep 9, 2010
    I haven't completely reverse-engineered one of those testers, if that's what you mean.

    But the concept is simple enough: Use the op-amp wired as a comparator to compare each test voltage to a reference. The output is either high or low, depending whether the test voltage is above or below the reference. A quad op-amp allows for 4 tests.

    For each voltage to monitor, you need to provide a reference voltage. There are a lot of ways to do this, including dedicated voltage reference ICs, zener diodes, or a voltage regulator. A common solution is to supply a regulated voltage to the entire circuit, and just use that as the reference.

    From the reference, any lesser voltage can be established using two resistors in series from Vcc to ground, to act as a voltage divider. The total resistance should be 10-100K to minimize current while also minimizing the effect of op-amp impedance on the measurement.

    If you want only one LED lit at a time instead of a "bargraph" style, you'll need to learn about window comparators. But I don't see why a bargraph wouldn't be fine for your application.