How do I make an LED turn on when my output reaches 5v?
A TL431 is about the cheapest and easiest way to do this. See below schematic. The reference voltage of the TL431 is 2.495V nominally, so about 5V and greater (it's divided by two with the two 1K resistors) will turn the LED on. 

If you are trying to detect a nominal 5V supply rail, you should set it to detect a lower voltage such as 4.5V to allow for tolerances. That would mean making R2 a bit bigger or R3 a bit smaller. 

Only about 3V is available for the LED so avoid using high \$V_F\$ types such as blue and white. 

The TL431 is a reference and an amplifier, all in a 3-pin package that costs only cents in quantity. 

A similar circuit to this is often used to regulate the output voltage of switching power supplies, where an optocoupler LED is placed where your indicator LED is. 









simulate this circuit &ndash; Schematic created using CircuitLab