In this instructable I'll show you how I solved the problem of my girlfriend having cold feet all the time.
Because warm shoes fit perfectly to the coldest time of the year, those shoes will be her christmas present:-)

I've seen quiet a few solutions to warm up your feet by the use of a heating element. But all of them are either rather expensive (e.g. heated soles, which contain a li-po battery warming your feet for a few hours), bulky (external battery pack which has to be attached to the leg somewhere), not very economically (pretty much every solution using batteries) or just unhandy (powering via USB-cable => I think pretty much everyone can see the attached laptop crashing on the floor...).

I decided for a nearly cableless solution. I added a heating element which is powered via inductive coils. So while you're sitting in your favorite spot you can put your feet on some kind of platform and your feet get warm, you can still move around freely and don't have to change or reload any batteries.

on the down side: Your feet are not warmed up while walking around.
But on the other hand: when you're walking around and working inside you usually don't freeze...

Step 1: Skills and Materials

Skills:

Very basic soldering skills (attaching cables...)
knitting and hand sewing (if you want to make the shoes really warm;-))
You should be able to cut with a knife, without losing a finger;-)

Materials:

two pairs of inductive chargers (I used those: http://www.adafruit.com/products/1407)
heating elements (I scavanged mine from two heated scarfs)
approx. 12V/10W power source.
approx. 2m of two-wire cable
A pair of house-shoes (you need some with a hard sole, so that you can integrate the inductive coils)
felt wool for knitting + 5 size 12 knitting needles
Yarn (to attach the felted house shoes to the hard soles)
plywood (~4 times the size of your foot)
glue
epoxy glue
wood glue
some thick paper
some laquer
exacto knive
Soldering iron
scroll saw or hand saw
needle nose pliers
small drills

Step 2: Knitting and Felting the House Shoes

I bought a set containing needles and the felting wool which also came with instructions and a size chart for the shoes.

I made size 36/37 (EU) shoes (~6 US/Women).
Here's the pattern:
Cast on 30 stitches
make 31 rows
close to a round, distribute the stitches on 4 needles, continue in rounds
Make 25 rounds
decrease by 4 stitches in every round until you've got 8 stitches remaining (should be another 7 rounds)
pull the yarn through the remaining stitches and make a knot.
Sew together the heels

Repeat for the second shoe.

When finished those shoes will look really big. Throw them into the washing machine, add a few tennis balls and wash them at 40°C.
When taking them out they should have approx. the right size and have a pretty dense surface (no holes anymore)

Step 3: Preparing the Soles and Adding the Electronics

I bought a pair of cheap house shoes and removed all the unnecessary stuff with an exacto (I'll put on the much warmer felted shoes instead).

Next cut a an approx 3mm deep circle out of the sole under your heel. It must have the size of your inductive coil. Don't make it too deep, because the the transmitted power will suffer a lot if the distance gets bigger!

If you try to keep the ring you cut out intact, you can glue it back on later (after making it as thin as possible) so, noone will even notice that there're coils inside.

Drill two holes through the sole, for the wires leading from the coil to the rectifier IC.

On the inside of the sole cut a rectangular whole in the size of the PCB for the rectifier. Make it as deep as possible, so the wearer won't feel it when walking.

Desolder the coil from the pcb, insert the coil inot the sole, insert the ic Into the other side of the sole and solder the wires back on (I made two cuts, so i could push the wires into the sole, to make them completely invisible).

If you took care while cutting the hole for the pcb you can use the cutout and put it back onto the ic.

First i tried to build the heating element myself by using wire, but it either got too hot or didn't warm up at all. So I decided to get some heated scarfs from the local super market (about 5€ per piece...). They contained small patches of thin hair like wire which are connected to a power source and get warm. Exactly what i needed.

I soldered the leads to the pcb and glued the patch onto the sole.

With everything attached, I ran a first test by putting the shoes onto the source coil: the heating element got warm after a few seconds:-)

Step 4: Building the Plattform

Perfect alignment is crucial for a high power transmission.
So i drew a template of the shoe on a sheet of paper to get the outline.
Then i took the paper, put in on the bottom side of the sole and traced circle where the coil was inserted.

Then i transferred the template to a 3mm thick piece of plywood.

I cut out the circle with an exacto and a chisel, just deep enough that it doesnt stick out. To get the wire to the outside, I cut out two small channels to the back.

To make the alignment of the coils as good as possible, I decided to add a small frame to guide the foot into the right position

I made a second template and cut it out on the inside using a scroll saw.

The two pieces were then glued together, perfectly aligned.

I then traced around the frame and cut out the glued together piece.

To attach the coils to the platform, you've to desolder them from the IC.

For the cables from the platform to the power source I used some old ear phone cables.

After inserting the coil, soldering on the cable and gluing the cables into the platform with some epoxy, I added a strong sheet of paper to cover the coil.

The earphone cables need to be attached back to the IC.

As a power source, I used an old 12V notebook power supply from my old and broken EeePC. I just soldered the power cable to the ICs and "mounted" everything with some duct tape.

To make the platforms look better, i added two coats of paint on the inside.

In the pictures you see a small rectangular cut out: First I wanted to use this for aligning the shoe with platform (+ a hole in the shoes), but the shoes sole were too soft and the alignment with the frame was already pretty good.

Step 5: Sewing the Felted Shoes to the Soles

To attach the felted shoes to the sole, I drilled holes around the sole, approx 5mm from the top at adistance of about 1cm. I used a 1mm drill.
I then sewed on the felted shoes to the sole with a thick needle and strong white yarn.

The very top of the shoes is quite hard to reach, so i needed to use some needle nose pliers to pull out the needle.
You also have to take care for alignment, it happens very easily that you sew on the shoe a little bit skew (happened to me, as you can see in one of the pictures... but they are still comfortable).

When finished, make a small knot between the shoe and the sole.

Step 6: Further Considerations

Power Consumption:
The inductive chargers have a maximum power of 500mA at 5v. That makes 2.5W under best conditions. Unfortunately that's not very much heat to make anything really warm, but in combination with the felted shoes, it helps getting cold feet to comfortable temperature.
Due to the low power any kind of burns or overheating should not appear.
As the felted shoes are rather thick it takes a few minutes for the heat to get through to your feet.

To get an optimum power output you need at least 9V of input. After some measurements with my lab power supply, I figured out, that power consumption is ~10W (500mA at 10V) with your feet on the platform. When your feet are lifted, power consumption is about 1W.

Automatic switch:
If I ever build another version I would add a push button to the platform, so that power is only transmitted, when the foot is placed onto the platform.

Summary:
I Presented you a cosy and comfortable way of keeping your feet warm without the hassles of recharing, changing batteries, or beeing connected to some power coord and ripping it out of the wall when trying to walk around.
Furthermore you've got some nice looking house shoes;-)

Have fun making your own!