I learned many tricks for working with cardboard and copper tape while debugging our prototype.
1. How to solder onto copper tape.
Tip: Use every option to keep your component still against the tape.
Work near an edge so that you can clip down on the copper. Temporarily tape down from the top with easy-peel scotch tape. Glue in place. Develop telekinesis. Whatever works.
Out of options? Break the soldering rules! Very, very delicately.
Heat up a bead of solder and tap it onto the tape. Using a non-flammable tool, press your component lead down into the solder. This cold join will keep it in place. Heat another bead of solder and tap it on top of the join. Gently press the iron over the edge of the join so that the solder flows together – but don’t let it get fully molten which might let the component pop out. Try not to press so hard that you puncture the tape.
2. How to plan your circuit
Tip: Minimize turns.
Corners are the enemy! Every corner requires a special accordion fold, and every fold risks breaking the tape.
Finally, remember your usual prototyping guidelines:
Prototype on a breadboard.
Check that your power source can really power all the things you need. A 1.5V coin cell battery cannot power two LEDs in series. That’s just life. A photoresistor may not work under the light conditions in your space – they need very bright light to conduct.
Pencil your circuit down before you put down the copper tape.
Place all additional elements on the board and verify the placement before you tape. Make sure you can easily access the tape in case of breaks – avoid covering your circuit.
Do you want a sliding switch? You need to build in stoppers and guides. The cardboard needs to be thick enough not to bend when moved. Ensure it will press tightly against the board if it acts as a conductive bridge over a gap in a circuit. This is in some ways the hardest part – cardboard warps easily and the copper must fully contact both copper lines in order to function. Bigger pieces of cardboard are less likely to bend. Plan your contact surface to be away from edges, which will start warping first.
Plan the display. If people pull or push on the card, what keeps it from falling over? Do you need to weight it?
Keep a circuit guide and multimeter on hand for repairs. Anything available to the public should be planned around easy repair. It will bend! It will break! Have a back-up and a Plan B at all times.
Bonus Guide: Advanced debugging
One circuit would not light its LED. We struggled to determine why.
1. It was in series with another LED and powered by a single coin cell battery. We shorted the circuit to test each LED separately. One worked, the one we cared about did not.
2. Tested for breaks in the copper tape. None detected, although a pile up of copper tape was removed just in case.
3. Visual inspection. We were working on the circuit side of piece, not the show side. I closely inspected the front for possible faults. Bingo!
Our LED was not working…because it was a photoresistor. Emit, absorb – they’re practically the same thing, right?
So in summary we swapped in an LED where it needed to be, and built a separate circuit with its own battery. And then spent another hour or so trying to make sure the copper tape slide made proper contact.
Spoiler: It did not. A problem for another day!