How-To: Embed a wifi detector into a backpack strap

Do you have a messenger bag or backpack you use regularly? Do you have a laptop or other device with wifi in said bag? Wouldn't it be cool to integrate wifi sniffing into your bag and not have yet another device on your keychain? Today we will show you how to take an existing inexpensive passive wifi detector and embed it into the strap of your messenger bag or backpack.

Creativity is the key with this hack! If you have basic soldering and sewing skills, you can finish this mod easily in an afternoon. A flickr stream with annotations for this project is located here. Images with flickr annotations are clickable (i.e. if the image is associated with the flickr stream it will link to to it).

The basic run-down of what we will be doing:
A. Opening up an existing wifi sniffer and desoldering the LED's.
B. Adding extension cables to the LED's so they can be used in the strap instead of on the device.
C. Creating a squishy switch to replace the wifi sniffer's "search for networks" button.
D. Modifying the LED's with small rectangles of plexi and attaching them into slits in the strap.
E. Restitching the strap and attaching the wifi sniffer in a corner of the bag.

Materials required:
- a backpack or messenger bag preferably with a padded strap (if you do not have a padded strap you can create one out of fabric and flat fabric foam)
- a cheap wifi detector
- some thin cables (ide ribbon cable can work well)
- some thin plexi scraps
- for the squishy switch: aluminum foil, brown paper (or similar), white glue, scotch tape, electrically conductive paint, and thin foam (fabric foam)
- super glue, a glue gun, soldering iron and a file
- optional: super bright LED's to replace the original LED's

Part A. Opening up the wifi sniffer and removing the LED's

First open your wifi sniffer. Ours is a cheap Connectland Wifi-n-Spy Finder but any inexpensive wifi sniffer with LED display will do.

Remove the LED's with a soldering iron. Note that LED's do have a direction, one lead is positive, one negative.

Part B. Add extension cables to the LED's

Measure the distance from the corner of the bag where your wifi sniffer will be attached and the position on the strap where you will place your LED's. We had three LED's on our wifi sniffer and therefore cut ourselves three paired cables. You may choose to use IDE cable for these extenders.

Solder the cables to the points where you desoldered the LED's on the circuit.

Solder an LED to the end of each extension cable. You may choose to replace the original LED's with super bright LED's. At this point test your wiring by pressing the "search for networks" button on your wifi sniffer (while in range of a wifi network).

Part C. Making a squishy switch

Now we are going to construct a simple squishy switch out of brown packing paper, foam and aluminum foil. This momentary switch makes a connection when you press the two aluminum foil sides to each other through small holes in the foam. This type of switch is used for interactive carpets and flexible switches integrated in clothing. We will be using the switch in place of the button on the wifi detector to trigger wifi network sensing in the device.

First glue the aluminum foil to the brown packing paper with white glue. Sandwich the foam between two sides of aluminum foil and cut to the desired size for your switch. Since this switch will be integrated into the strap of your bag feel free to choose the size and shape of your switch.

Cut a paired cable for the switch from ide cable or similar thin cables. This cable should be the length from the position of the switch in the strap to the position of the wifi sniffer in the corner of the bag. Strip one end of the cables back about two centimeters and tin them with solder. Pierce two holes about one centimeter apart in the foil/paper for each cable and thread the tinned end through from the paper side to the aluminum side and back again. Fold back the remaining end. Pinch the cable to make as much contact between the tinned wire and the aluminum foil. Daub a bit of conductive paint on the wire on the aluminum side to maximize the connection. Repeat this for the wire on the other side of the switch.

Tape a border of scotch tape all around the edge of each side of the switch on the aluminum side to keep the switch from making contact on the edges. Reinforce and tape over the wires on both the aluminum side and the paper side (after the conductive paint has dried).

Cut holes in the foam (about one small hole per square centimeter seems to work well) custom fitted to the shape/size of your switch. Tape the three layers together along the edge. Test the switch with a multimeter.

On the circuit of the wifi sniffer find the two contact points to which the "search for networks" button is connected. You can find this by trying to short out the two points with a small piece of wire. Our contact points were two diagonal points to which the button was soldered. Solder the two wires from your squishy switch to these two points.

Part D. Rectangular plexi "lenses" for the LED's

We cut small thing plexiglass rectangles through which the LED's will shine in our backpack strap. You may choose to simply use rectangular LED's or place your LED's behind clear plastic windows on your strap instead.

Cut a notch in each small rectangle of plexi and super glue the led to the indentation.

When dry, add a layer of hot glue on both sides to reinforce the connection.

Part E. Embedding the rectangles in the backpack strap

Cut open the edge of the backpack strap by carefully cutting the thread stitches. Cut a slit for each led/plexi rectangle. Pull all the cables up into the backpack strap. Push each plexi rectangle through the slit and affix it with a thin line of super glue.

When pulling the cables, you will be much more comfortable if the cables are on the side of the foam of the backpack strap that won't be in contact with your shoulder. With everything in place, stitch the backpack strap back closed.

Attach the wifi sniffer in place in a corner of your bag where it can have the best positioning of the antenna. We sewed a small pocket into the corner of our messenger bag where the wifi sniffer's antenna is pointing up and away from us (blue rectangle in the image below). The squishy switch can be embedded into the bottom edge of the strap of your messenger bag on the underside with some fabric and foam (red rectangle in the image below). The yellow zig zag on the blue rectangle below represents the position of the wifi detector's antenna; it should be pointing up and out to sniff networks. Alternatively, you may choose to embed the squishy switch into the padded part of your backpack strap or along the upper edge of your bag.

Flickr annotations of all the photos for the How-To can be found here.

Be sure to check out more cool hacks, How-To's, and DIY projects on Hack-A-Day. Enjoy your embedded wifi sniffer and send us links to photos of your versions to the tips line!