I have an omnipolar (2ssm) hall effect sensor which outputs 400mV when triggered by a magnetic field. Basically what I want to know is, is there a way to increase this output voltage to be able to power an LED?
The aim is to have an array of these setup to detect flux leakage over an area and have the LED's light up in the area the flux is detected.
Since you just want to operate LEDs that designate an on / off indication you may not even need to use an op-amp for this as suggested in the comments. Instead setup a circuit that uses a comparator device. These can be found with four per package. Set one input of the comparator to a reference level of say 350 mV. The Hall sensor output feeds into the other input of the comparator. The output of the comparator can drive the LED with suitable series resistor. What the comparator will be showing is if the Hall sensor voltage is above or below the reference voltage.