There are many kinds of conductive epoxies, glues, and rubbers available. Unfortunately, they are for the most part quite expensive.
So, for more than four years, I have been trying every possible combination of likely elements, compounds, and solvents that I could get my hands on, in order to make my own truly affordable conductive glue. While I have found several that have excellent low conductivity, they tend to be quite brittle and have a tendency to crack. The conductive glue presented here does not have these problems.
Because this glue is quite flexible and you can vary its resistance, it has potential for different kinds of paint on sensors for robots or other devices. It should be possible to paint on strain gauges on the outside of a regular glove and use it for virtual reality or other control possibilities. Touch sensors and membrane switches can be painted on various flexible or rigid surfaces.
It can be used to paint on wires and resistors and as a glued solder joint. It can also be used to paint on strain gauges, temperature sensors, electromagnetic shielding, antennas, and push-button switches. I suspect, that with more experimentation, it may be possible to use it to create capacitors, diodes and transistors.
Step 1: Materials for the Conductive Glue and Circuit
Carbon Graphite, fine powder-Available in larger quantities at http://www.elementalscientific.net/
Available in smaller quantities at your local hardware store. It's called lubricating graphite and comes in small tubes or bottles. The brand I used successfully is called AGS Extra Fine Graphite, but no doubt there are other brands that will also work.
Performix(tm) liquid tape, black-Available at Wal-Mart or http://www.buytape.com
Mixing cups or glass container
1/4 and 1/2 teaspoon measuring spoons
Glass or plastic mixing rod
Cardboard for stencil
Toluol paint thinner (optional)-Available at most hardware stores.
Conductive thread (optional)-Available in larger spools at http://members.shaw.ca/ubik/thread/order.html It is available in smaller spools at
Circuit materials of your choice
Step 2: Mixing the Glue
WARNING: This process involves nasty solvents in the mixing process. Be sure to mix and apply either outside in the shade or inside in a VERY WELL VENTILATED AREA. Avoid contact with skin until the solvents have completely evaporated (The next day).
Step 3: Applying the Glue
Like most conductors, the resistance of the glue is roughly proportional to the cross sectional area of the conductor. In this case, a glue line about 1/4" wide by about 1/16" inch thick will have a resistance of about 32 ohms per inch. While this is not that good for running power to the main circuit, it quite fine to run digital signals or to light LEDs. By comparison, an equal amount of "Wire Glue" has a resistance of about 90 ohms per inch.
Step 4: Creating Components
See fig-B. To create a strain gauge (a resistor that varies resistance as it is bent), you can lay down a coat of liquid tape on a piece of flexible plastic. When it is partially dry lay down a coat of conductive glue between two wires or pieces of conductive thread that are about an inch apart. After that has dried, lay another coat of liquid tape over the whole thing. The strain gauge in the picture has a resistance between 70-300k ohms as it is slightly bent.
The strain gauge or a glued resistor can also be used as a temperature sensor. If it is taken from the shade into the full sun, the resistance increases. So, it could be used to measure the difference between room temperature and overheating conditions.
To glue two wires together, if you can, you should first twist or crimp them together and then coat them all the way around with conductive glue. If you merely lay them side by side without touching and then glue them, the resistance of the glue joint will be in 7-9 ohm range. See fig-A if you are using conductive thread (resistance is about 2 ohms per inch) for wires, you can tie an overhand knot to the wire you are connecting to and then coat all around it with the glue.
In fig-C you can see that it is also possible to glue surface mount LEDs.
Step 5: Glue a Complete Circuit
In this circuit I used conductive thread to provide power from the batteries. Conductive thread is flexible and glues quite well, but you could also used tinned wire. Magnets in front and back, hold the conductive thread to the two batteries and act as a battery holder.
Step 6: Possibilities
While I have experimented with at least 157 different chemicals, metals, solvents, or elements, I have not tried most of them with the liquid tape. So there is ample room for those of you who like to experiment to improve upon the mix.
For those of you who would like to see a movie of this simple circuit that uses a Picaxe controller to blink 4 LEDs in sequence, you can download an video file at: http://www.inklesspress.com/glued_circuit.wmv
Step 7: Update
Thank you for your feedback in the comment section. This is an open source conductive glue so I tried some of your suggestions and the results are:
The lubricating graphite resulted in a resistance equal to the graphite from a chemical supply. However, it is quite a bit more expensive for the amount of glue you get. If you just want to see if it works or only need a small amount of glue, it works fine.
Wire Glue works well. Even though it is the cheapest of the conductive glues and epoxies that I have seen, It is more expensive and has a higher resistance than this open source glue. It also is brittle and will crack if you bend it. You cannot as easily vary its resistance. But it works just fine if you only need small quantities. On the plus side, it has no fumes and sticks very well to glass. You can obtain it at:
Metals Instead of Graphite
I have had mixed results in my experiments with powdered metals. While they often produce the lowest resistance per inch, they tend to make the resulting mix quite brittle. They also considerably increase the price of the glue mix. If you would like to see a list of the metals I have tried and the results along with the latest update on Conductive Glue Experiments, you can see them at: http://www.inklesspress.com/conductive_glue.htm
My original goal was to come up with a glue that was less expensive, had a lower resistance, and was more flexible than the nearest comparable commercial product. While there is room for improvement, this glue is as useful as most of the off the shelf conductive glues. As for me, I am quite happy to continue experimenting with the proven graphite formula which has low cost, high flexibility, variable resistance, and many possibilities.