I have a few RC planes and I want to use them upgrade them for night flying.
I would like to add EL Wire and/or Arduino + NeoPixels. I am more of a software guy than an electrical engineer so I am struggling with some basics.
Using an extra battery to drive my circuits is too heavy. I need to tap into the existing battery to get power.
EL wire typically uses an inverter that is powered with 3V (2 AA's) My Arduino setup is using Adafruit Trinket 5V and 5V NeoPixels.
What is the best way with little weight to draw 3 & 5 volts from 3.7V (1s), 7.4V (2s), and 11.2V (3s) LiPo RC batteries?
Another big question is amperage. I understand too little is a no go but, when is too much a concern?
The best way is to use a switching regulator, stepping down from 7.4V or 11.1V (depending on what voltage battery you are using). Don't tap into the battery to get a lower voltage, as this will unbalance the cells.
Small lightweight switching regulators designed for RC models are readily available. They are usually called a 'BEC' (Battery Eliminator Circuit) and are commonly rated for 3A output - more than enough for your purposes.
To avoid having to use another regulator to get 3V, get an EL inverter that is designed to run from 5V (eg. Align Driver For Cold Light String BG71011A). Alternatively you could just stick with LEDs, which are easier to use and more reliable. To get the best effect for night flying I recommend shining the LEDs onto parts of your model rather than pointing them outwards. That way you get better coverage with fewer LEDs, and a softer light that won't blind you!
If your model runs from a single 3.7V cell then you probably need to keep it as lightweight as possible, so just run everything directly off the battery. If you must have 5V to power an Arduino etc. then use a step-up regulator (eg. TURNIGY Voltage Booster for Servo & Rx (1S to 5v 1A)). I'm guessing you probably don't want to do that though, because models powered by a single cell are usually so light that the extra weight will seriously affect their flying ability.