Prep Work

Optional but recommended: using a tiny bit of 5-minute epoxy or E6000 glue, reinforce the point where the wires connect to the battery. Allow to dry completely before continuing.

I’ve gotten in the habit of doing this the moment new batteries arrive, before even starting projects with them.

Using a hobby knife or a pointed file, scratch away the trace between these two points on the LiPoly Backpack board.

This enables use of a power switch, rather than being always-on.

If making a pendant, confirm that each of the electronic components fit in their corresponding spots inside the case without undue force. Use a file to scratch away any protruberances that interfere.

Optional but recommended: splay the “wings” of the switch just slightly so it fits more snugly into its spot inside the case.

Also, I like to trim the legs (using flush cutters) to about half their normal length. Not required, but gives just a little more working room inside the case.

Wiring & Soldering

Any wire lengths mentioned here are for the pendant assembly. If making a votive or other holder, it’s fine (and probably helpful) to use longer wires, whatever length you need.

For reference, here’s that circuit schematic again of what we’re aiming for:

Cut two pieces of wire about 1.5" (40mm) long and strip and tin both ends.

Tin two legs of the switch and connect wires here. Use the center leg and then either one of the two outer legs; the opposite one won’t be used.

Solder the opposite ends of these two wires to the LiPoly Backpack board, to the pins which had the scratched-away trace between them.

Trim away any wire protruding from the underside of the LiPoly board. This is especially important for the pendant…it needs to be nearly flat. A slight protrusion is a normal product of soldering and will fit inside the case, but gloppy soldering here will cause trouble.

Adding heat-shrink tubing to the switch legs before soldering to the LiPoly board is totally optional, but I like to do this for durability. Use it if you got it!

Snap off a 3-pin section of row pin header and solder it to the pro Trinket’s BUS, G and BAT+ pins.

The LiPoly Backpack is installed over this…BUT…instead of sitting atop the Pro Trinket like normal, turn the board over (keeping the same pin alignment…see photos) and solder it hanging off the side.

Make sure the header is perpendicular and the boards are aligned. Start with just the middle “G” pin, re-heat as necessary to get everything aligned, then solder the two outer pins. Let the solder flow into the vias, do not glop it up on the surface!

After soldering, trim away the pins protruding on both sides using flush cutters. We want this as flat as possible.

Snap off two 9-pin sections of row pin header. Solder these to the A1-A9 and B1-B9 pins on the Charlieplex driver board as shown. A solderless breadboard can be handy for holding these straight while soldering. The other 8 pins don’t require connections.

Set the 3D-printed spacer piece on the back side of this board, then lay the LED array on top of it. Hold it snug with a rubber band, tape or a couple clothespins, then solder the pins on this side.

The spacer is required for the pendant (the battery will fit into this space later). For a votive candle or other design, you can sandwich the boards directly without the spacer…unless you want the battery in that spot.

After soldering, use flush cutters to trim the pins on both sides: the LED face and the component face.

BE EXTREMELY CAREFUL AS YOU DO THIS. Do not clip off any LEDs! Also, tiny bits of metal will go flying…wear safety glasses and/or point it away from you when cutting…and make sure those bits don’t end up landing in the circuit somewhere.

For a really low profile, an alternate method is to solder only the middle pin on each side, trim all the pins flush, then finish the rest of the soldering.

This requires really ace soldering though…properly-tinned iron, heating each spot, adding just a little more solder and letting it flow in. If you’re heavy-handed or a “melt-and-wipe” solderer, that isn’t gonna cut it. Watch Collin’s Lab: Soldering for pointers!

Cut four more wires about 1.5" (40 mm) long, then strip just a small amount of insulation and tin each end. They don’t need to be all color-coded like this, but it does make things easier…use it if you got it.

On the back of the Pro Trinket board is a spot for an optional JST connector. Tin the + and pads there, then solder two of the wires to those pads as shown.

The other two wires connect to pins A4 and A5.

These four wires then connect to the matrix driver as shown in the circuit diagram or using the table below. These wires snake up from “between” the LED/matrix sandwich and are soldered on the component side, then trimmed flush.

Pro Trinket Pin

Matrix Driver Pin

+ (JST pad on back)


– (JST pad on back)






If building pendant: push the spacer out from between the boards using a small screwdriver, then slide the battery into this space in the direction shown. Plug the battery into the LiPoly Backpack board.

For non-pendant builds: the battery doesn’t need to fit there unless you want it…just plug it into the backpack.

Test Run

Flick the switch to the “on” position and see what you get! It’s normal for the Pro Trinket to flash its red LED for about 5 seconds, then the sketch will start, and you should see the animation playing out on the LED matrix.

If it does NOT run:

  • Is the battery charged? Try plugging in USB to power the circuit that way.
  • Did you upload the FirePendant sketch to the Pro Trinket board as explained on the “Software” page?
  • Double-check the connections between the two boards. Did you get SDA and SCL crossed?
  • Look around for any cold joints, solder bridges, or bits of conductive detritus that may have fallen in when clipping pins flush.

If everything works, switch it off and we’ll finish sealing this up.

If it still doesn’t work…first, confirm you can get the basic Arduino “blink” sketch uploaded. That’ll help confirm whether it’s a hardware or software problem. Then you can ask for help in the Adafruit Forums. It’s extremely helpful if you can provide a couple well-lit and in-focus photos that clearly show all the connections.

Do not continue until you have the flame animation playing on the LED matrix.

This guide was first published on Apr 15, 2016. It was last updated on Apr 15, 2016.

This page (Soldering) was last updated on Apr 03, 2016.

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