Ever since the Palm Pre came out I’ve been in love with it’s inductive charging upgrade. Similarly to the Powermats, the charging base acts as a device stand with it’s integrated magnets in addition to charging your device without having to plug and unplug your device every time you need to use it. Once on the charger it outputs 5.50v. It’s a bit higher than other USB chargers I’ve used.

Inductive charging back with base

“Induction chargers typically use an induction coil to create an alternating electromagnetic field from within a charging base station, and a second induction coil in the portable device takes power from the electromagnetic field and converts it back into electrical current to charge the battery. The two induction coils in proximity combine to form an electrical transformer.” -Wikipedia

After peeling off part of the backing you can see the coil and it’s output circuitry. You can also see the four magnets it uses to attach itself to the charging base. They do a good job of holding my heavy phone and case to the base.

After finally getting in the Touchstone base and replacement back I decided to get this working over some lunch. I was a bit uncertain at the time that the magnets would be able to hold my phone while it’s in it’s case. I also wanted to make sure that I wouldn’t experience any signal loss issues or suffer from any overheating while charging. I found the quickest way to make this work was to take the male end of a microUSB plug and wire it directly into the contacts off the inductive back. After taking apart the microUSB plug from a spare dead cable, you can use a 200 ohm resistor to short the D+ and D- pins on the plug. This allows the device to charge at a higher current, which will in turn charge a bit faster. After soldering it up I used some colored silicone to insulate the back of the plug.

Roughed in and adjusted the case to improve clearance

It works!

In order to clean this up and make it a bit more permanent and presentable, I have get away from using the charge port and get it through the housing. Unfortunately, the back housing of my phone has a nice metal frame right where I want to mount it. There are also a few antennas I need to avoid. The induction coil is going to have to stay on the back of the case, and somehow get passed thorough the metal housing into the board. I drilled through the back of the case near the sim card holder.

It took a bit of poking and prodding but I was able to find 2 test points that when the motherboard was plugged into a USB charger had +5v showing between the points and the ground plane. No magic smoke was released!

After a bit of soldering I was able to route the positive lead out of the phone housing and is now staged for the next step- going through the housing and into the case. The metal frame of the sim card holder is attached to the ground, so I can use that to complete the circuit. On a normal USB type charge port the ground pin attaches to the ground plane which is usually where all the shields are anchored into so I feel safe doing the same.

For the next stage of the project I’ll have to use some contacts or pogo plugs embedded within some silicone to go from the inside of the phone to the contact points on the induction charger. Ben had the good idea of using some thin copper prototyping board to act as thin traces.

To be continued!