Homebrew USB Charger

There are plenty of examples on the net on how to build your own usb charger, but since we didn’t have any on our site I figured I’d share it with you.  I found an old wall wort in my junk drawer that I couldn’t remember what it went to, so I figured it would be great to put it to a good use as a USB device charger. Because I didn’t have a clue what the maximum current draw requirements were for a USB device, I found the requirements on USB.org’s website.

Difficulty : MEDIUM - SAFETY WARNING, Shock Hazard
Time: 1 hour

The victim

To start off with I cut off the existing dc output wire of the wall wort and then cracked open the case (If you’re more patient you can probably get the wall wort open without destroying the plastic case, but I ended up putting it in a vice to crack it open). From there I de-soldered the AC wires that are attached to the AC outlet prongs and removed all the electrical innards from the plastic case. After that, I de-soldered the transformer output wires from the circuit board so I could re-locate the board after I installed it into its new box.  Since the voltage regulator is only rated for .5 Amps decided to remove and replace it with a 1 Amp LM7805 regulator. I ended up attaching a heat sink to it to keep the regulator cool, as shown below.  Btw, the transformer is rather over sized so I’m not worried about pulling too much power through it.

Next, I found a used project box to put the transformer and circuit board into. I didn’t re-use the original enclosure because I destroyed it while opening it up. I found an old beat up ac cord and an extra panel mount USB connector, then I drilled two holes into the project box to make room.  I soldered the new ac cord in onto the transformer and then glued the transformer into the project box with hot glue.  To get the stepped down voltage from the transformer to the circuit board, I ran another set of wires from the other end of the transformer to the circuit board.

AC to DC converter

Everything mounted in the box

After the circuit board was placed inside of the box I got to work on soldering the USB connector to a small proto board, shown above.  I decided to add the proto board to make mounting and connecting a couple of components much easier (plus when i tried to solder wires and components on to the USB connector pins directly I was running out of room and started creating shorts).  On the proto board I added a 1 Amp PTC (thermal resettable fuse) in series with the positive wire from the output of the AC to DC converter circuit and the output 5 V of the USB connector.  The PTC will provide device protection without the need to ever open the box and replace a fuse. I also decided that I wanted add a power indicator on the circuit, so a green led and a resistor in series with the 5V output was installed.  I installed the green led by drilling a hole in the top of the enclosure and used an clear-ish epoxy to mount it (I have to admit that the led is a bit dimer than I would have liked through the epoxy).   In order to charge the iPod correctly, the USB specification states that the data puts must be shorted together. As a precaution, I soldered a 51 Ohm resistor between the data pins on the usb connector. This way the pins are pulled together with low resistance rather than a short.  To finish things off, I attached the ground wire from the circuit to the ground on the usb connector and was ready to test.

Ipod Charging

Above, my 5th gen i pod video attached, it drew about 700 mA which was well within the limits of the supply hardware.

With the project complete I’ve got a functioning USB charger to keep at work which cost me nothing to put together (much better than paying $30 for one).

USB Charger Complete

Side view of charger

It is pretty rough looking, so I just hide it behind my computer monitor =).

Update (8/19/09):  Here is the circuit diagram for the charger.  Enjoy!

Circuit Schematic

GPS-500 Setup

Looking for a cheap GPS receiver? Look no further than the GPS-500 Microsoft Streets and Trips GPS by Pharos. This tiny unit features a SiRF Star III chipset and most are selling on ebay for just under 10 dollars! This GPS receiver can be interfaced via a RS232 serial link using a MAX232 chip from Maxim IC and a low cost USB to serial converter.

Difficulty : Medium
Time: 1 Hour

GPS-500 Side

GPS-500 Top

After purchasing this GPS receiver, I thought that it would be ready to go, very wrong. In reality the unit needs a special converter cable to interface with a serial port, who has time to buy and wait for that?

Assuming that most devices use 5 volts, I started the very dangerous method of applying power and ground to various pins on the GPS receiver. Once the power was applied correctly a blue led turned on.

Now, I just needed to get the NMEA output data from the GPS receiver. Using a RS232 converter like the MAX232ACPE shown in the schematic below, I connected pin 14 of it to the RX pin of a serial port and listened for data via hyperterminal. Next, I attached a wire to pin 11 of the MAX232ACPE and tried the remaining pins on the GPS receiver.

GPS Test Schematic

Upon finding the TX pin of the GPS receiver, hyperterminal was receiving garbage text because of an incorrectly set baud rate. After some trial and error, the baud rate was found to be 4800bps.

Based on what I have found, the pinouts are as follows below. The unused pins could be RX or some sort of special functionality that the SiRF Star III GPS chipset has to offer.

GPS-500 pinout

Lastly, I added a switch on the output of the GPS’s TX line, a usb to serial converter, and LOTS of hot glue to secure the wires in order to interface with my laptop. The switch was added to shut off the GPS’s TX output from sending data to the computer during the first 5 seconds of powering up. Otherwise, hyperterminal would cease to read from the device.

Finalized GPS Schematic

The SiRF Star III technology is quite remarkable, I was able to acquire 8 GPS satellites on the first floor of a two story house!

GPS Teardown

GPS Laptop Setup