I put this together in a bit of a rush, but I'm sure someone else can do a nicer job.
And it's my first instructable!!
Step 1: Materials and Tools
3mm Diffused LEDs
A switch (preferably the smallest / thinnest you can find)
A couple of 1.5v button batteries (I used AG-10)
Double sided tape
Step 2: Make the Circuit
Strip some wire and join the batteries together and link them to your switch and to a couple of bits of foil stuck down with double sided tape.
Step 3: Add the LEDs
Tape the LEDs to the foil "runners" remembering to check the polarity!
Step 4: Assemble the Card
Cut a heart (or anything else you fancy) out of another piece of card, and cut a square of plastic from the clear plastic wallet. Tape this over the front of your card / circuit and enjoy.
Write something nice on the inside and send!!