More and more portable devices connect to their chargers via mini-USB 2.0 connectors.  In this article I’ll show you how to build a car charger for your Blackberry, Pocket PC, phone, PSP, bluetooth headset, etc. out of junk you probably already have lying around your house.

USB Car Charger

The theory behind this mod is simple; a 7805 voltage regulator drops the 12-13.8 volts coming out of your car’s cigarette lighter receptacle down to the 5 volts required by USB devices.

Parts Needed:

  • Cigarette lighter plug (the car charger from that old analog cell phone you’re tired of looking at will do nicely)
  • USB cable with mini-USB 2.0 connector on one end (so many devices come with these now you’re bound to have an extra somewhere)
  • 7805 +5V Fixed-Voltage Regulator (you’ll probably have to go buy this one. Radio Shack has them for $1.59)

Required Parts

Tools Needed:

  • Soldering Iron, solder, etc
  • Screwdriver (if needed to take apart your cigarette lighter plug)
  • Needle-Nose Pliers
  • Wire Cutters
  • Utility Knife
  • Multimeter
  • Hot Glue Gun

Start by disassembling the cigarette lighter plug. Remove all the guts and set them aside.

Plug Guts

Cut or desolder tip and side contacts from circuit board if needed. You should now have a hollow shell and two sets of contacts.

Plug with Contacts

Next you’ll need to prep the USB cable. You need to find out which wires goes to the power pins on the mini-USB connector. Most of the time it will be the red and black ones, but some cables may be different. If this is the case, cut the insulation away from a section of the cable, but don’t cut any of the wires yet. Plug your cable into a computer or charger or something that will put power on the cable. Use your meter to locate the wires that have 5 volts across them. Also note the polarity.

Use a meter to find the correct wires

Once you know what two wires you need to connect the 5 volts to in your USB cable trim off any excess wires and connectors. You should now have a cable with a mini-USB connector on one end and two wires sticking out of the other.

Now it’s time to start soldering everything together. The way your cigarette lighter plug is built and how much room you have in it will determine exactly how everything is laid out, but electrically you should end up with this:

Schematic

If you have any parts or contacts that just won’t stay put because you yanked out the PCB that was supporting them or they are just being difficult, use some hot glue on them.

(It’s hard to see all the parts in this picture; use the schematic to figure out how to connect everything)

Everything Soldered Together

If you want, you can install an LED in the cigarette lighter plug. Hook it up in series with a 250-ohm resistor across the 5v output of the voltage regulator (pin 3) and ground (pin 2).

When you’re done, all the components should fit inside the case of your plug and all the contacts should be in their proper places. The USB cable should come out the end of the plug where the original one did (bonus style points BTW for using the stock strain-releif). If everything looks good go ahead and put the plug back together. Since no external modifications were made to the plug it should look like it was born as a USB car charger.

Reassembled

Take your charger to the testing lab (car). Plug it in. If you installed an LED, look and see what it does. If it doesn’t come on at all you probably didn’t hook something up right. If it gets really bright for a second then melts, you probably didn’t hook something up right. If it comes on and stays on, congratulations, you have hooked up the voltage regulator correctly.

This next step is a test of bravery. If you are brave you can connect one of your precious portable devices to the charger and see if it charges. If you are not brave (or are perhaps just smart) you can test the voltage coming out of the mini-USB connector. If it reads about +5v, go ahead and plug it in!

The red light means it’s working!

Tested

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