Fabricating from flexible PCB

A project log for Flex PCB Solar Motorized Earring

A tiny, solar powered, motorized spinner, made from a flexible PCB by folding into a rounded cube. An electronic wearable, an earring.

Lloyd KonnekerLloyd Konneker 07/18/2019 at 23:220 Comments

The boards arrived and I fabricated one.  See photos.  Flexible PCB from OSHPark is rather pretty.

It folds nicely.  I bent the tabs that fit in the slots at right angles.  That makes them stay in the slots without glue.  You can bend the PCB severely, at less then the radius recommended (about 2mm) and the bend will more or less stay.  In my case, there is no circuitry on that portion.

I gingerly bent the sides down around a 1mm rod (an impromptu mandrel.)

I spent much time debugging, only to find that I had oriented and installed the solar cells backwards.  Solar cells are diodes.  But the current they generate is in opposite direction to the current you would put through a normal diode.  That is, in the opposite direction of the arrow part of the diode symbol.  Remember, I am using photodiodes as solar cells.  So the base of the arrow is the positive side of the diode as a current/voltage source.

I usually draw an "orientation" diagram for all the parts that need orientation during fabrication.   But I got it wrong.

And, on the KiCad footprint I made,  I omitted a diode symbol on the silkscreen  that would have helped me orient the solar cells.

Also, it turns out the footprint I made was not correct.  So the part pins barely overlapped the lands. They did reflow, but are likely to suffer more soldering failures than usual.   I will need to revise.

Also, the TPS3839 footprint is wrong.  I think it should be SOT23-3 (a TI name?)  instead of SOT-323?  Again, the part pins barely overlapped the lands on the PCB.  In a revision, I think I will use a TI "DQN" package, a tiny 4 pin package for a single transistor.

Finally, the earring does turn, but as noted before, it requires bright light:  at least outdoor shade with much exposure to bright sky.  A better design would use a little bit larger solar cells.  But again, they are not easily available.