I have made a simple adder. It adds two single bit numbers and outputs a result. My adder works like expected, but the problem is that I want to light a led as the output. When I get a current of 20 ma at the led with a 150 ohm resister the current the voltage source outputs increases from about 20ma to 40 ma. I want to keep the current at around 20ma to increase the battery life. How can I achieve this?