Sometimes, a visual indication of whether a sensed voltage is above or below its nominal value can be useful. Most approaches to over voltage or under voltage sensing use two voltage comparators and a resistor divider to form a window comparator. The circuit in Figure 1a is an alternative to the traditional window-comparator approach. It provides different-color indications if the sensed voltage is above or below the preset value; in this case, it is centered around 0V.
The circuit uses a FET-input, low-offset-voltage OPA124 op amp and a dual-color LED. The forward voltages for the red and green LED sections are 2 and 2.1V, respectively. The values of the op-amp feedback resistors R1 and R2 are such that the op amp's closed-loop gain, 1+R2/R1, equals VLED/VWIN, where VWIN is the desired positive or negative window threshold. Thus, whenever the input voltage, VIN, exceeds ±VWIN in magnitude, the op-amp stage supplies a voltage that turns on the corresponding LED. When VIN>+VWIN, the red LED turns on; when VIN<–VWIN, the green LED turns on. Whenever –VWIN
<+VWIN, both the red and green LEDs are off. R3, typically 5 kV, limits the maximum on-state LED current. You should choose R1 such that the feedback current through R3 is small compared with the on-state LED current. You can ignore the small difference between the red and green LED forward voltages for most applications, or you can balance it by adjusting the op-amp offset voltage. 
For asymmetrical window voltages, you can use the configuration in Figure 1b. In this case, you assume |VWIN–|>|VWIN+|, where |VWIN–| is the magnitude of the negative window voltage and |VWIN+| is the magnitude of the positive window voltage. Q1 is an NMOS enhancement-mode MOSFET that has a threshold voltage of approximately 1V. The source terminal of Q1 connects to the negative input of the op amp; thus, it remains at a virtual-ground potential. The gate terminal connects to the op amp's output, which turns Q1 on whenever the output voltage exceeds Q1's threshold voltage.
For input voltages greater than 0V, the op amp produces a negative voltage and Q1 turns off. The ratio of R2 and R1 sets the op-amp gain, and the output clamps at the on-state voltage of the green LED, approximately –2.1V. For input voltages lower than 0V, Q1 turns on once the op amp's output exceeds the threshold voltage of Q1. In this case, the ratio of R1 and the parallel combination of R2 and R3 sets the op-amp gain, and the maximum output voltage is the on-state voltage of the red LED, 2V. Resistor R4 again serves as a current limiter for the LEDs. The relationship between the resistor values and the positive and negative window voltages is given by the following equations. For simplicity, we use only the positive magnitude of the voltages, and we neglect the difference between the forward voltages of the red and green LEDs.
You should choose the value of R1 such that the feedback current through R4 is small in comparison with the on-state LED current. Choose R3 such that its value is much greater than the on-resistance of Q1. The op-amp configuration in Figure 2 has resistor values that set the VWIN– window at –5V and the VWIN+ window at 0.8V. For the case in which |VWIN–|<|VWIN+|, you can replace Q1 with an equivalent PMOS enhancement-mode MOSFET. When the window voltages VWIN– and VWIN+ have the same polarity, you can also use the circuit in Figure 2.
This circuit inserts a unity-gain difference amplifier (for example, an INA105) in the front end of the circuit in Figure 1a. This added stage subtracts a reference voltage, VR. You can use this type of window-comparator circuit to monitor a power-supply voltage, such as 5V, with preset limits of 4.75 and 5.25V, for example. The following equations yield the window voltages: Source: Mark Stitt, Burr-Brown, Tucson, AZ
