The first set of fairy lights I purchased a few years ago had a very rudimentary embodiment. Just two AA batteries, an on/off switch, and a string of miniature LEDs on enamel wire.
I love the simplicity of the design... The design works well enough with disposable alkaline batteries. However recharge cells don't work very well on their own, as their combided state of charge voltage very quickly drops below the forward voltage of the white LEDs.
Fairy Tech 2.0
Browsing at the dollar store today, I found some new fairy lights which included some circuitry on a PCB. What could it be?
Same general look and feel as before.
Now included is a three position switch!
The mysterious PCB
No sign of an inductor anywhere 
. Just a
- 8-pin MCU?
- Watch crystal
- SPDT Slide switch
Fairy Tech 3.0
I can upgrade this new set of fairy lights with a boost converter module capable of operating of a single alkaline/Ni-MH cell
Module based on ME2188 https://file.elecfans.com/web2/M00/64/78/poYBAGMHOg-APsG0AAge4TQOpd0961.pdf
I bought a value pack awhile back thinking I would receive a selection of different output voltages. Nope, I got a bag of identical modules and each has a nominal output voltage of 5V. So, I added an additional 50 Ohm series resistor to limit the current to approximately 40 mA.
I am taking a bit of a gamble assuming the MCU is 5V rated. If it gives up the ghost, I'll replace it too.
All for now.
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