They're terribly expensive for something so simple
https://www.instructables.com/id/10-BICYCLE-USB-CHARGER/
The Sinewave ones major expense is the machining of the unit. Look at their matchbox style one, it’s about half the price.
@Eeyorecore started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
They're terribly expensive for something so simple
https://www.instructables.com/id/10-BICYCLE-USB-CHARGER/