Intro: Printing Circuits With Conductive Paint
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
The circuit is pretty simple, it centers around a 555 timer IC. The 555 timer is wired up in astable mode, meaning that it will be outputting a continuous square wave, with a frequency dependent on the resistance between the two pins labelled "POT" in the diagram above. The only extra components needed are a 1kOhm resistor and a 0.01uF capacitor. The circuit is powered by a single 9V battery, connected at the ground and 9V pins. A speaker is connected to the ground and output pin (pin 3) of the 555.
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
First I drew out my schematic in Eagle and used the board layout editor to design the traces for the PCB. I purposefully laid things out so that the length of the traces between components was as small as possible. When using this Bare Conductive paint it's important to remember that it will add some resistance to your circuit, using shorter traces minimizes the extra resistance introduced to the circuit from the paint. After I arranged everything the way I liked I increased the width of the traces to 0.076" (this was the max width I felt I could get away with). Again, this was to minimize resistance from the paint; thinner traces will have more resistance, so if you want to avoid this you'll have to make your traces as wide as possible. I found that my traces had a resistance of about 600ohms per inch (you can check out the Bare Conductive datasheet for more info). I've attached my Eagle files below.
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
I used tweezers to carefully remove the traces that were just cut from the tape mask. To transfer the mask without distorting it, I covered the exposed regions with a supporting piece of blue painter's tape, peeled off the whole mask, laid it down on paper and then peeled the supporting tape off.
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
I pressed firmly on the edges of mask to prevent paint bleed through, then I covered the mask with a thick layer of Bare Conductive paint. I used a paintbrush to spread the paint evenly and made sure to cover all the exposed parts of paper. The paint took about 15 min to dry. Once dry, I carefully peeled off the blue tape mask with tweezers.
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.
Step 5: Prepare and Attach 555
I bent and trimmed the legs of a 555 timer (fig 1) so that it fit nicely on the traces I'd just painted on the paper (figs 2 and 3). I used the paint pen to place a dab of paint on each lead of the 555 timer. This helped to secure an electrical connection to the traces and glue down the chip to the paper.
Step 6: Add Components
I used a sharp piece of wire to poke holes in the paper for my components (figs 1 and 6). I threaded the leads of each component (one 1k resistor, one 0.01uF capacitor, and one jumper wire) through the holes and bent the wires back on the other side of the paper (much like what you would do if you were about to solder to a real PCB). I used the paint pen to add a dab of conductive paint where the components met the conductive traces on the paper. Once dry, I cut the excess wire from the back of the paper.
Step 7: Draw Variable Resistor Traces
This is the part I wish I had pictures for! In this part of my project I drew parallel, freehand traces leading out from pins 7 and 8. When shorted, the traces form a variable resistor (because of the variation in length between the short, and where the traces connect to the 555 timer). Be careful not to make these traces too long or too thin, you'll find that the resistance will become too great, and the circuit will not work.
Step 8: Add Battery and Speaker
Wire up the battery and amplifier as close as possible to the pins of the 555 timer, you do not want to add extra resistance to this part of the circuit. I even added a dab of extra paint for physical reinforcement and increased conductance. Now stick a piece of copper tape on your finger (I found this was necessary) and connect the long traces to change the pitch of the 555 timer. If you're having trouble producing sound, try adding extra paint to your traces to decrease the resistance, changing the values of the cap and resistor, or using an oscilloscope to debug.