- Powdered graphite
- White vinegar
- Elmer's clear glue
To make the ink, put powdered graphite in a cup, cover with vinegar, and stir. Let it sit for a few minutes.
Once the graphite settles on the bottom of the cup, remove the clear liquid on top with a syringe.
Stir in about a teaspoon of glue to keep the graphite suspended. A thick line of paint has a resistance of a few kilohms per inch.
To test, use a small brush to draw lines connecting a 3-volt coin cell battery to an LED. Let the circuit dry and watch it light up!