There are already technology using carbon or silver particle ink like "Bare Conductive" or "Circuit Scribe". But they are expensive, high resistance. Here I introduce a way to build circuit on any transparent or translucent surface with thin copper wire, UV glue, regular components, solder wires and soldering iron.
When thin copper wire is glued to glass, it has very low resistance and it is almost invisible. All other parts can be glued to glass in same fashion. When they are soldered together, circuit will work as good as they are on a regular circuit board.
Step 1: What Is UV Glue
UV glue is a kind of transparent liquid adhesive. It flows like syrup but it will solidify in seconds when it is exposed to UV light. This characteristic makes it ideal for us to adjust position of parts and wires until we are satisfied. Once ready, we can apply UV light to joint and the glue will solidify quickly. The glue will still be transparent after cured. The best thing is, heat from soldering will not destroy the glue.
Step 2: A Simple Circuit, a Touch Activated LED
Here I started with a very simple touch activated LED circuit. Once you touch between 2 pads, your finger will introduce small current. That tiny current will be amplified to drive the LED.
I'm using surface mount parts to build the circuit but through hole parts will also work. Resistors values doesn't need to be accurate and you can use any NPN transistor you have.
You can find the circuit on:
Step 3: Parts and Tools
coil cell battery holder, 2 NPN transistors, 2 resistors, 1 LED.
soldering wire, stranded wire, flux, UV glue.
any kind of glassware.
Disposable container, Q-tip
wire-cutter, tweezer, soldering iron.
Step 4: Prepare Parts
Before we glue parts on glass. We should coat all pads with solder and flux to make following soldering easier.
First we add a bit solder on all pads of all parts, then we apply a thin film of flux on all pads. The flux will give us a nice finish when we solder wires on those pads.
Step 5: Glue Parts on Glass
First we should pour some UV glue into a disposable container. A few drops will be enough.
Then we can use Q-tip to apply some glue to back of coin cell holder, fix it in correct position with thumb ,and use UV flashlight on it's side for 10 seconds. Once the coin cell holder is fixed, we can apply UV light on the back of coin cell holder for another 20 seconds to fully secure it.
For small parts we need to apply glue first, then use tweezer to place them in place. If you can lie down the glassware, stickiness of UV glue can hold parts in place. So you can pick and place multiple parts and cure them with UV light together.
Step 6: Connect Wires
Thin copper wire can be obtained from stranded wires. We can strip end of a stranded wire and pull out the thin wires out of the plastic coat. Then we will get a bunch of low-resistance thin copper wires.
First we should solder one end of the thin copper wire to a pad. If you don't get a smooth finish, add some flux to that pad and solder again. Once the copper wire is soldered, you can start thinking about how to route the wire. If bending a wire is necessary, we can bend the wire, hold the bending point in place and some UV glue to that point. If you are satisfied with that position, apply UV light to secure the wire. When routing is finished, solder the other end of wire to designated pad and cut excessive wire with a blade or wire cutter. Repeat the steps until all components are connected.
For the touching pad, we are going to use two wires in parallel. Bend and route the wire just like other wires. When you reach the end of wire, apply small amount of glue to secure it in place, cut the excessive wire and apply more glue to secure your wires.
The final step is to add a drop of glue on all small components to protect them, and add a few drops of glue behind the battery holder because you don't want it to fall off when you replace the battery.
Step 7: Add Battery and Make It Work
When everything is finished, put battery into the circuit. Press between 2 wired and watch the glass glow.
Step 8: Use Same Circuit on Flower Pot
I've also built the same circuit on a flower pot with electrodes inside. When the plant is watered you will see the happiness of the plant.