This will show you how to make a 12v to USB (5v) adapter. The most obvious use of this is for 12v car adapters, but anywhere you have 12v you can use it! If you need 5v for anything other than USB, simply skip the steps about adding the USB ports ;)
You will need
- a fuse holder
- a 0.5 amp fast blowing fuse (you can use a higher rated one if you use a different transistor that can take higher amperage. Because we put the fuse on the 12v side, (which can vary from 11.5-12.5 volts, we have to use a value 2.5x smaller than what we want on our USB side. So, if you want 1.5 amps for your USB ports, then you select a 0.6amp fuse, if you want 2.5 amps at 5v, you select a 1 amp fuse, if you want 3.75 amps, you select a 1.5 amp fuse, etc.). Also, if you really want to protect your circuit badly, just put one on both sides.
- 2 different colors of wire (so that you don't get confused later on)
- a L7805CV voltage regulator, part number 497-1443-5-ND from Digikey (search for 296-11414-5-ND if you want higher AMP output, like 2 amps since it is a max of 3 amps along with a 1-1.2 amp fast blowing fuse, and LT1084CT-5#PBF-ND if you want 2 ports at 2 amps each, along with a 1.5-1.75 amp fast blowing fuse.)
- a 220uf 16v Capacitor, part number P5139-ND from Digikey.
- female USB ports times the number of ports you want (2-3 would be normal, you can expect most devices to use up to 0.5A each, except for tablets that can use 1-2 amps), Digikey number AE9923-ND
- if you choose to add an LED, you need an LED and an appropriate resistor for 5V. This depends on your LED's rating. Any LED will do, here are the values for the most common LEDs : 1.2v = 220ohm, 1.6v = 180ohm, 2v = 180 ohm, 2.2v = 150ohm. If you have a strange LED or want to make it brighter, please use this resistor calculator.