One way to get into night flying is to fill a plane with LEDs. You can do that by connecting them directly to the motor battery, or to a dedicated second pack, but neither approach gives you any control. Here's how to reuse an old servo to drive your LEDs, with either on/off or variable brightness from the transmitter.
1. Start with a standard size servo, perhaps one with stripped gears or a dead motor. Smaller servos will work, but won't be able to drive as many LEDs as they're designed for less current.
2. Remove the servo horn, and then remove the main screws which hold the case together. On some servos you may need to cut a factory sticker which is holding everything together.
3. Remove the top case and the gears. You might be able to put the finished LED driver back in the servo case, but to save weight I tossed it.
4. Remove the amplifier board from the servo case. I needed to apply pressure on the motor and the potentiometer from above as shown here.
5. Once the components are free, it's not a bad idea to plug them into a receiver and make sure the motor spins. You could also try an LED across the motor terminals.
6. Desolder the motor from the circuit board. It may be attached directly or by wires. If there are wires and they're long enough to be useful, just snip them at the motor and skip step 8.
7. Cut off the potentiometer, making sure to note the resistance. This pot is labeled 5K ohms.
8. If necessary, solder red and black wires to the motor terminals. These will be connected to the LEDs.
9. Make a W shape from two resistors as shown. I chose a 2.2K and a 4.7K resistor to simulate the pot being set to one side. If you use unequal resistors, try them both ways with an LED attached to see which gives the right polarity matching the red and black leads.
10. Solder the resistors onto the three posts from the pot. The twisted middle wire must go to the center post.
And there you go! Shown here is a 3S LiPo pack connected to a 25 amp brushless ESC, plugged into channel three as normal. The LED driver is connected to channel 6, which corresponds to the flaps knob on my Futaba 7C . Because the resistors are unequal, I had to reverse the channel and then set up the end points by trial and error. The result is the LEDs are completely off when the knob is fully counter-clockwise, and they come up to full brightness as the knob reaches 12 o'clock.
Alternatively, you could use two equal resistors, each about half the resistance of the original pot. In that case, the LEDs would be off at 12 o'clock, and could be setup to hit full brightness at the fully clockwise position. Either way, use a voltmeter to determine how many volts the circuit is putting out, and then calculate the resistors for your LEDs accordingly.
To finish up, I'd recommend covering the LED driver in shrink wrap for protection. I'd also install a micro Dean's connector or similar to the LED leads for ease of installation. Finally, remember that this driver is powered by the receiver, so you may want to use a separate BEC which can handle the current. The built-in BEC found in many ESCs cannot handle more than four servos when stepping down from 11 volts.