I wanted a 5 volt source for my newest gps, a Garmin Quest. It has a internal battery that is not intended to be service by the consumer. The Quest is recommended for automobile. A speaker is in the car adapter and using the Quest in the car the is a constant power input of 5 volts. You can remove the Quest for a walk and it will runs from 6 to 20 hours on the internal battery, depending the mode of uses. When plugged into the car, the internal battery in recharged. On my bicycle, I have 12 volts 2000 ma batteries packs for my lights. For the Quest, I want a 12 volt to 5 volt voltage regulator that uses the least internal current. I made this unit and is testing it now.
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also machine the adapters and battery/light holders shown in the application link. Machining Plexiglas and pvc was fun chip making.