[DonRotolo]

In addition to what was said before, some other considerations:

1. Be sure to consider the power dropped by the resistor. As an example, because P=I*V, 40 mA at 18 volts = 0.72 watts - use a 1 watt or larger resistor or it'll overheat quickly.

2. For LED dropping resistors, close enough is fine. If your calculations call for 310 Ohms, anything from 270 (a bit broght) to 390) a bit dimmer) is fine.

3. You must always have a resistor in series with LEDs, or they will go into hard conduction and overcurrent very rapidly, letting out a tiny pink glow and a small pop as they fail forever. Start too large and move smaller.

4. LEDs are diodes, too, so running two in opposite polarity, but in series, will mean no current flows.

The short answer to what you did wrong is, your 110 Ohm resistor is too small, Al gives good values.

Don