This is an instructable on the best conductive glue I have been able to make that uses easy to obtain materials, glues well to most materials, is quite flexible and has a low resistance. Unfortunately for me, it uses a rubber or plastic mixture that I do not know the chemical composition of, so I cannot patent it.
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.
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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
After some experimentation, I have found that mixing the Carbon Graphite and liquid tape in a 1-1/2 to 1 ratio by volume, produces the lowest resistance. If you add more graphite than that, the painted line tends to become more brittle and can shrink and crack. An easy way to mix is to use several mixing cups and add 1-1/2 spoonfuls (using a 1/4 teaspoon) of the graphite into each cup. You can then wipe the measuring spoon and then slightly overfill it with the liquid tape. Pour and scrape this into one of the cups and mix up a batch. It is best to mix up one small batch at a time as it tends to dry very quickly and can get gummy fast. Mix it fast and be sure to get all the carbon mixed in well before you use the glass rod or a toothpick to apply it to the circuit you are creating.
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
If you want to apply it in one coat You need to apply the glue when it is almost the thickness of peanut butter. You can use the Toluol paint thinner to thin the mix if it starts to get too gummy or if you want to paint it on thin. Toluol can dissolve some plastics, so you should mix it in a glass container. The thinner the coat the higher the resistance you will get. This is ideal for painting resistors. Generally, if you paint on a second coat you will cut the resistance in half. Up to a point, each successive coat will reduce the resistance of the conductive line or resistor.
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
You can create a line stencil by cutting a 1/8" or wider slot in a piece of cardboard and painting in the slot. To create resistors you should first test your mix between two wires taped an inch apart. Let it dry overnight before you measure the resistance. Even when slightly wet, the resistance will be higher. To create a resistor I use a mix of 1/2 graphite to 1 liquid glue or of 1 to 1. To increase the resistance add less graphite to the mix or make the line less wide or thinner or longer.
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
Here you can see a circuit that was glued onto a clear plastic cover. This simple working circuit uses a Picaxe microcontroller to flash 4 lights in sequence. It is used only to illustrate the concept, obviously you can choose your own integrated circuits and make the circuit of your choice. Be very careful when gluing onto the IC pins as the black glue can be very hard to see if it gets on the black IC and can end up shorting the pins.
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
This conductive glue is a fraction of the cost of any other conductive glue including "Wire Glue". You can mix it up in large quantities. As you can see, I have barely scratched the surface of what is possible with an inexpensive conductive glue and glued circuits. Perhaps these circuits could be silk-screened on. Maybe one could paint glue on a surface and then use a laser to remove where you don't want it to conduct and etch a circuit that way. Circuits can be constructed on various 2 and 3 dimensional surfaces including paper, cardboard, plastic, fabrics, and glass. As mentioned above, many components such as capacitors, diodes and transistors might also be constructed.
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.
RyanB215 made it!