After making a flexible circuit, I wanted to experiment with other ways of working with conductive paint. I saw this project posted by the people who make Bare Conductive paint and I had the idea to try to create a circuit on paper using lithography. This is just a sample project I tried, it's a finger activated 555 synthesizer. You could use your own parts and circuits with this technique, though you need to be aware of how the increased resistance on the traces will affect the electronics. I made this project a while back, and unfortunately seem to have misplaced my final images, so this documentation is a little more about the technique than the actual project.
Bare Conductive Paint Pen Amazon
0.01uF ceramic capacitor Digikey 399-4324-ND
1kOhm 1/4watt resistor Digikey CF14JT1K00CT-ND
555 timer Digikey LM555CNFS-ND
9V battery clip Radioshack 270-324
copper conductive tape Amazon
amplifier and speaker
22 gauge jumper wire
Blue painter's tape
Step 1: The Circuit
I also included a jumper wire in my circuit, indicated by the two pins labelled "jumper" in the schematic above. A jumper wire is a wire that connects one part of a circuit to another in case it is too difficult to find a path for it on the printed circuit board (PCB). Pins 4, 5, and 8 should all be connected to 9V, but since they are on opposite sides of the 555 timer this was a little tricky. As you can see in fig 2, I already had conductive traces running under the 555 timer, so I had to use a jumper cable to connect pins 4 and 5 to 9V without running into other traces or taking a very long path around the chip (although this might not be a problem with regular copper traces, a long path with conductive paint will add some unnecessary resistance to the circuit).
Step 2: Design in Eagle CAD
I exported an image of the traces to Photoshop and made a few adjustments (increased the size of some contact pads). I imported my final design into Illustrator and used the live trace function to generate a vector file of the traces. I've attached the final EPS file below. I laid down some blue painter's tape on a piece of acrylic and cut the PCB vector file into the tape with a laser cutter. If you don't have access to a laser cutter, you could try printing the EPS file and using it as a guide to cut the mask by hand.
Step 3: Transfer Mask to Paper
Be careful not to adhere the tape to the paper too well, you could end up ripping up the top layer of paper when you remove the mask later. After a few failed attempts, I found that it worked best to stick the tape to my hand a few times before putting it down on the paper, this way it wasn't quite so sticky.
Step 4: Paint and Remove Mask
Inspect the traces for discontinuities and shorts here. Patch up any broken traces with a dab of paint and rip up any short circuits with a razor blade if you can.