Here is a fun and easy soft circuit project. I took an Instructables' Robot t-shirt and added some LEDs for the eyes and a flashing red LED for Robot's heart. The switches are situated where the grey buttons on Robot's front panel are.

The circuit is sewed onto felt that is attached to the inside of the t-shirt with Velcro so it can be taken off and the t-shirt can be washed.

Step 1: What You Will Need

Instructables' robot t-shirt (you can win one, or buy one here)
Felt: red and white for the heart and another piece for the circuit panel
Velcro
Matching thread
Pins, needles and scissors
Needle nose pliers
Washable fabric glue
Tape

Circuit:

Conductive thread (I bought mine here)
LEDs for the eyes (I used some yellow ones which I got from cheap set of battery powered Christmas lights)
Two push button switches
Resistor (use this site to determine which one you need)
Button cell battery
Flashing red LED circuit ( this I got from cheap flashing dollar store toy or you can make one)

Step 2: Prepare T-shirt: Eyes

You will need to cut holes in your t-shirt for the eye sockets. Place your L:EDs over the eyes and gauge how big the holes need to be. It is better to have the holes fit snugly around the LEDs, if they are too big the LEDs will slip out of the sockets when you move around.

Step 3: Prepare T-shirt: Heart

Robot tends to bear his heart only around Valentine's day, but I wanted this t-shirt to have more going on than just LED eyes so I added a beating (flashing) heart. Robot's heart can be seen when a chamber is open on his chest. So you will need to cut a square with flaps opening out on both sides.

Step 4: Prepare T-shirt: Heart Continued

Cut a small heart from red felt slightly smaller than the opening you made in step 3. Cut a square piece of white felt about 0.5cm larger (on each side) than the opening in step 3.

Using washable fabric glue, glue the heart onto the centre of the white felt and glue the white felt onto the t-shirt (gluing it on the inside of the shirt with the heart facing out).

Step 5: Felt Panel

Now we need a piece of felt to sew the circuit on. Just cut a squarish/rectangular piece of felt roughly the size of Robot. The colour doesn't matter as it will be on the inside of the shirt.

With fabric chalk mark on the felt panel where the eyes, heart and buttons are to be situated. Use a ruler to measure, you'll need it to be fairly accurate.

Cut a small piece of felt slightly larger than the battery, this is for the battery holder (see step 10).

Step 6: Prepare Circuit

To connect standard LEDs and resisters in a soft-circuit it is best to curl the legs with needle nose pliers into loops so that it is easier to tie the conductive thread to them. Do the same with the prongs of the push button switches.

Lay out your circuit components onto the felt panel and with fabric chalk so you can sketch out the circuit paths. Make sure the paths that you will sew with conductive thread are not too close to each other or criss-cross since this would cause a short circuit.

Step 7: Beating Heart

My flashing circuit had little holes around the edges (probably for screws) so I used them to sew it onto the felt panel by just looping regular thread through the holes and felt several times.

The on/off mechanism of this circuit was a spring sitting above a plate, when the circuit was bumped the spring would hit the plateand the LED would flash. However, I wanted it to be turned on with a push button switch so I pulled the spring away from the plate and tied conductive thread to it.

With a running stitch I sewed along the path leading to the switch and tied the thread to one of the prongs of the switch.

Step 8: Beating Heart Continued

With a new piece of conducive thread tie the end to the other prong of the switch and sew a running stitch to the plate of the flashing circuit. Here I just taped the end of the thread to the plate, making sure that it has good contact with the plate.

Using regular thread secure the switch to the felt panel so that it doesn't move or shift.

Step 9: Eyes

This is really a normal circuit with a switch but instead of wire and solder you use conductive thread.

(BATTERY + )------>---( SWITCH )------>---(RESISTOR)------>---(LED +)---->---(LED +)
(LED - )------>---(LED -)---->---(BATTERY - )
.
With one end of conductive thread stitch through the negative loop of the far LED and through the felt. With a running stitch connect to the second LED by stitching through the negative loop and the felt.

Stitch along the path until you get to where the battery will sit and loop through the felt several times to make the negative connection node for the battery (see image below). From here do a running stitch to the switch and loop the thread around one of the prongs of the switch, tie off thread.

Step 10: Eyes Continued

With a new piece of conductive thread connect the positive loops of the LEDs as you did in the previous step. With a running stitch move along to the resistor and tie the thread to the loop of the resistor and tie off.

Take a new piece of thread and tie the end to the other loop of the resister and with a running stitch move along to the switch and tie the thread to the other prong of the switch. From here sew a path to where the battery will sit. Take the small piece of felt that you cut for the battery holder in step 5 and loop the thread through the centre several times to make the positive node.

Sew the battery holder to the felt along three sides with regular thread, slip the battery into the holder and sew closed the fourth side. Make sure the battery fits snugly and there is good contact between the thread nodes and the poles of the battery.

Secure the switch to the felt using regular thread.

Step 11: Finish Circuit Panel

Test your circuit so that everything is working properly. Sew two piece of Velcro (the half with the hooks) onto the felt panel. One piece should sit directly above the eye LEDs. The second piece (also hooks) about 0.5-1cm below the switches.

Step 12: Velcro on T-shirt

Position the felt panel on the inside of the t-shirt so that the eyes, heart and buttons line up and pin the felt in place.

Turn the t-shirt inside out and mark where the other Velcro halves should go. Glue the two Velcro pieces (these should have the loops) onto the t-shirt. Be sure not to get too much glue on it because it can seep through to the other side and stick everything together. Once dry (check bottle label to see how long it takes), attach the felt panel to the t-shirt, turn it right side out and your done!

Extra button
As you can see, Robot has three buttons on his front panel, so if you felt inclined you can add another component. I was thinking an audio recording of some sort would be a nice addition.