Here’s the schematic again, for reference:
When building two or more poi, you’ll probably find it faster and easier to do each step on all the poi, assembly-line style, rather than building each one start-to-finish.
It’s vital thoughout this project that the solder flow smoothly between components. Cold solder joints — where the solder is beaded on the surface — do not withstand physical shock.
Fresh solder flux (which is built into the core of the solder) is essential. You’ll have one or two tries to make the connection…any more than that and the solder starts to get sticky and misbehave. When this happens, you can mop up the bad solder with your iron and a solder sucker or copper desoldering wick, then begin again with virgin, flux-rich solder.
Why are two different gauges of wire being used?
Heavier-gauge wire has less resistance and can carry more current…but space is so limited here we can’t fit the 26 gauge wire throughout. So the battery and power-hungry LEDs will get 26 gauge, while the microcontroller and signal wires (which don’t require a lot of current) use the slimmer 30 gauge.
Try to keep the wires orderly on these last few steps…use tweezers to “comb” the wires apart as needed so they’re not twisted around each other.
Solder the mode button’s longer wire to Trinket pin 3.
The narrower (30 ga) ground wire then solders to the Trinket’s Gnd pin.
That’s all the soldering on the Trinket and backpack! There’s still more to do elsewhere though…