After opening the speaker set i went about verifying the voltage output of the speaker's wall adapter. It was odd since the adapter claims an output of 9V @ 400mA. My meter would show almost no voltage coming from the adapter. Yet it is able to power the system. So i went into my box of "warts" and pulled out one rated 9V @ 500mA. This adapter actually puts out about 15V without a load. The speakers seem to handle it fine with nothing overheating. Besides the power will be divided between the speakers and charging when an iPod is docked.
Next i went about building a charge circuit for the iPod dock. I used a very basic circuit using an LM7805. I quickly found that you need more then just 5V going to the power leads of a USB cable to get an iPod to charge. So i found a great instructable on How to get your iPod to charge with your homemade charger
. So i broke out my breadboards so i could prototype the circuit to ensure i get it right. There seems to be allot of hit and miss with different resistor combos people have used. Since I didn't have the mentioned resistor values on had i broke out the trust Voltage Divider calc on my iTouch and went about finding a good combo using what i had. From the instructable i knew i needed 2V going into the D+ (green wire) and about 2.7V going into the D- (white wire). I ended up with the following based on the 5.03V output my charge circuit offers:
- R1= 22K Ohms
- R2= 26K Ohms (1x 22K resistor and 1x 4K resistor in series)
- R3= 15K Ohms
I hooked everything up then went about testing the circuit before connecting it to my iTouch. Making sure sure that the D+ (green wire) was at 2V and the D-(white wire) was at 2.7V. I found that the diagram on the above mentioned instructable has the voltage divider upside down. R1 needed to be coming from the DC input and R2 and R3 need to be attached to ground. Otherwise the voltage readings across the data pins wasn't reading right. So i connected everything as my schematic shows and all the measurments worked out. Since everything looked good i plugged in my iTouch and it worked without a problem. Even with the voltage across D- at only 2.27V. I am using a mini-USB B cable like those found on many cell phones. The iPod connector i'm using is from a charging kit and is an adapter that plugs into a mini-USB B cable.
Before moving everything to a board i wanted to hook it up to the speakers and see how the wall adapter handled the load from the speaker and the charger. Everything stayed powered with room to spare. The output voltage of the charger was 5.03V without a load. Once the iTouch was connected it only dropped to 4.93V. Overall not bad for such a basic circuit. Since everything checked out i was ready to solder the charger to a board. But not everything can go onto the board yet. The power input and USB wires need to be left off till final assembly.
After laying out the components on the board i soldered everything down and ran wires over the top to where i wanted the USB cable to connect. Since the wires from the USB cable are so thin i used a healthy helping of hot glue to secure then to the board so they won't move and break. I did a quick test to ensure everything still worked as it should the the charger is complete.