9

Smart backpack with wireless charging pocket

TUTORIAL

DESIGN YOUR CIRCUIT

Start by working out where all of your components

should go. It’s best to place the light strip close to

your Trinket, and the Trinket close to your charger,

so that wires don’t have to pass across any zip

openings. A front pocket is a great choice for housing

the charger and the Trinket, and the LED strip works

well on a flat area, like the top flap of the pocket. The

wireless charger should be able to charge through

one or two layers of thin fabric, but do a quick test

by holding the phone and charger in place before

committing to a location. Take into account the risk of

pickpockets in your local area when deciding where

to put the pocket.

MAKE THE LIGHT STRIP

Measure the NeoPixel strip against your bag to

determine how long it should be. Leave about 1 cm of

space on each end for the casing to attach to the bag.

Use scissors to cut the strip to the correct length. Cut

directly through the contacts. It’s a good idea to round

the corners so they don’t snag on your fabric.

Note the direction of the small arrows on the

NeoPixels: they point in the direction that data flows

through the strip. Cut three pieces of silicone-coated

stranded wire about 7–8 cm long, and solder the wires

to the ‘Data In’ side of the strip (the side the arrows

point away from). Apply a bead of hot glue over the

solder points for reinforcement. We’ll cut the excess

length off of these wires before we solder them to

the Trinket.

Cut a strip of the cotton casing so that it is 4 cm

longer than the NeoPixel strip. Fold 1 cm of the strip

back on one end of the casing and sew it closed. Insert

the ‘Data Out’ end of the strip into the casing so that

the wires extend out of the open end of the casing.

Gently fold the wires toward the back of the casing,

making sure not to bend at the solder points. Be very

careful not to damage the NeoPixel strip during this

step. Sew down the fold with a few hand stitches on

either side of the wires as shown.

MAKE THE POCKET

Lay out your stretch fabric right side down, and place

your phone on top. Trace around your phone with a

marker. For a good connection, we need to make sure

the phone does not move around inside the pocket, so

trace tightly around your phone for a good fit.

Add 2 cm to each side for seam allowance, and cut

out your pocket. To keep the edges from unravelling,

zigzag over all four edges with a sewing machine; if

you don’t have a machine, you can run a bead of fray

check along the edges and let it dry. Fold each edge in

by 1 cm and stitch in place ½ cm from the edge. Check

to make sure your pocket is just about 1 cm bigger than

your phone on all sides. If it looks good, set it aside and

move on to making elastic straps for the charger.

MAKE A STRAP FOR THE CHARGER

To hold the charger tight against the front of the bag,

we’ll make an elastic strap for the charger to slide

into. This strap will be sewn into a ‘T’ shape, and be

attached to the inside of the bag, directly behind our

new phone pocket. Cut a piece of elastic that is about

4 cm wider than your charger is, and lay it across the

centre of your charger. Cut another piece of elastic

to extend from the middle of this piece to about 4 cm

beyond the bottom edge of your charger. Use a lighter

to carefully melt the cut edges of the elastic to keep

them from unravelling.

Next, pin the pieces together in their ‘T’

configuration. To sew the pieces together securely,

stitch in a rectangle shape, then sew an ‘X’ inside the

rectangle. Check that your strap will fit over the charger

with at least 1 cm of elastic on each side for attaching

to the bag.

Above

Measure the strip

against the bag to

know where to cut

Below

You only need to connect one end of the strip to control

all the NeoPixels

YOU’LL NEED

Backpack or bag

Portable Qi

charging bank

Fabric for pocket

(find something

with stretch that

co-ordinates with

your bag)

Waistband elastic,

5 cm wide

Hand-sewing

supplies

E-6000 or other

strong, flexible

glue

Trinket M0

microcontroller

NeoPixel strip

about 30 cm

¾” wide white

cotton bone

casing

Silicone-coated

stranded wire

Scotchgard,

or other

hydrophobic

spray coating