Flexible Circuits

I used conductive paint to make a simple flexible circuit that lights and dims an LED depending on how much it is flexed. I used to work on a project that involved fabricating circuits on contact lenses, where we used micron-precision lithography to lay down tiny conductive traces on the surface of flexible plastic. When I came across Bare Conductive's conductive paint, I had the idea to use lithography to make simple, flexible printed circuit boards for regular, through-hole components. I printed this circuit on transparency film, but this same technique could be applied to many other materials- paper, fabric, drywall....

Parts List:
Bare Conductive Paint Amazon
Resistive Flex Sensor 10-40Kohms Jameco 150551
10Ohm resistor Jameco 2161043
6.8KOhm resistor Jameco 691067
Amber LED Jameco 2006781
9V battery snap Amazon
Blue painter's tape Amazon
Transparency film Amazon
Hot glue

The Circuit:

This circuit has a 6.8Kohm resistor and the flex sensitive resistor in series to form a voltage divider between 9V and ground. When the flex sensitive resistor is flexed it changes its resistance. This causes the voltage at the junction between the flex sensor and the 6.8K resistor to fluctuate.

The output from the voltage divider goes into the base of an NPN transistor. In this circuit the transistor acts as a voltage follower. The purpose of the transistor is to match (well, nearly match, but we don't have to worry about that for these purposes) the voltage at the emitter to the voltage coming into the base (from the voltage divider). This way the voltage at the emitter is approximately equal to the voltage at the base, but all the current going from the emitter and into the LED and resistor is sourced directly from the transistor's collector instead of from the voltage divider. Basically, the transistor acts as a buffer, protecting the voltage divider from the load of the LED and its current limiting resistor.

So as the flex sensor is bent and the output from the voltage follower fluctuates, the output from the transistor's emitter will also fluctuate along with it. This causes the LED to increase and decrease in brightness. More specifically, the LED will glow dimmer the more the flex sensor is bent.

Steps:

I prototyped this circuit on a breadboard first. The resistor values I used on the breadboard were slightly different than what I used in the circuit because the bare conductive paint adds resistance into the circuit. On the breadboard I found that 3Kohm and 68ohm resistors were ideal in the voltage follower circuit drawn above. In the final product I increased the 3K to 6.8K and decreased the 68ohm to 10Ohm to increase the amount of current going through the resistor.

The fabrication process was simple lithography. I designed my lithography mask using Eagle, Photoshop, and Illustrator.

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 circuit would fit nicely alongside the 4.5" flex sensor. After I arranged everything the way I liked I increased the width of the traces to 0.085" (this was the max width I felt I could get away with). When using this Bare Conductive paint it's important to remember that it will add some resistance to your circuit, 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 think if I was to redesign this circuit I would minimize the distance between the transistor, LED, and ground in order to increase brightness. I would also remove the resistor in series with the LED completely because the traces themselves can act as a current limiting resistor for the LED. I've attached my Eagle files below.

I exported an image of the traces to photoshop and made a few adjustments. I deleted parts of the traces that I felt were too close to each other and increased the size of the component contacts where necessary. 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 sent this file to a laser cutter and cut my lithography mask out of some blue painter's tape. 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.

Once cut, I carefully removed the insides of the traces with a pair of tweezers (fig 4) and transferred this mask onto a piece of transparency film (fig 5). I diluted the Bare Conductive paint slightly with water and painted it on top of the lithography mask (fig 6). Once dry (after about 15 min) I carefully removed the tape with tweezers. I patched any broken traces with a dab of paint. I drilled holes in the film for the components using a drill press (fig 7).

Once drilled, I threaded my components in and used a dab of paint to cold solder them to the circuit (fig 8). Once dry, I gently clipped the excess leads. I added a bit of hot glue around the connection to the 9V battery snap to give it a little structural support. I also glued down the flex sensor to the surface of the circuit board.

Future:

I'd like to continue working on this technique, as it's quite less time consuming and wasteful than traditional PCB etching. I liked working with the transparency film because the end result looks interesting, but I think it would be better to work with a more hydrophilic material like paper- the paint would bond much more easily. I may also experiment with laser etching small wells in acrylic and filling these wells with paint- the added height might decrease resistance in the traces.