Touchstone charging on Nexus S 4G with NFC preserved

[darrenf]

Touchstone charging on Nexus S 4G with NFC preserved

This is a follow up to an earlier thread I posted here:

http://forum.androidcentral.com/spr...-webos-palm-pre-nexus-s-4g-18.html#post914626

in which I moved the inductive charging coil from a Palm Pre Touchstone-compatible back into a NS4G to allow for inductive charging.

At the time, I didn't appreciate that NFC would be of much use, but given today's announcement from Google about their new payment system, and given the excellent work by the NFC Task Launcher developer, I decided that I really wanted NFC after all.

First I put the NFC anteanna back into the phone but its location in the back of the phone puts it between the Touchstone and charging coil in the phone, preventing Touchstone charging. I theorized that perhaps the opposite wasn't true -- perhaps I could put the NFC antenna closest to the phone and the touchstone coil outside the phone and have both of them operate correctly.

I had to wait for the arrival of some NFC tags to test my theory. Those arrived yesterday and my tests reveal that indeed the NFC antenna, operating in it's original location, will work with and will not impede operation of an inductive charging coil running on the *outside* of the phone.

Last time I tried to post a long article it was held pending moderation, so this time I think I'll tell the story with photos.

I hope that my over-documentation makes up, in some small way, for the fact that I didn't photograph the connections that I made to the USB port internally.

If I had it to do over again I would make one change. I would drill or cut a larger notch in the back of the phone so that I could solder to the contacts on the charging circuit while it's attached to the outside of the phone. That would let me attach the coil more permanently to the back. I would like to put some CA glue on the charging circuit assembly where it contacts the back of the phone so that it will take up the stress of being removed from a touchstone, rather than the vinly doing all that work.

I know this mod isn't for everyone. One thing working in my favor was that I found the phone to be way too slippery and was already using vinyl stickers or electrical tape on the back to give me a better grip on the phone.

A side benefit of the mod is that the phone sticks to the Touchstone really well now, even in the car where I have it mounted more vertically than when it's on the desk.

Feel free to ask any questions!

-darren

 

[clindner]

Wow, that's a lot of work! I think I'll keep plugging mine in...

 

[drewsonian]

This is pretty sweet... but I'm not great with this kind of thing... Maybe someone will come out with a manufactured back that is compatible with the touchstone?

 

[darrenf]

Wow, that's a lot of work! I think I'll keep plugging mine in...

It would only take an hour or two, even if you had never done this before. I was driven by two motivators: 1.) Coming from the Pre I have Touchstones sitting around everywhere including in all my vehicles, and 2.) my son's Evo stopped charging because the micro USB port was fatigued to the point where it came loose from the PC board. I confess that I'm not all that careful with the phone, even when it's connected to a USB cable for charging, so by not using the USB port for normal charging I am extending the life of that connector (barring a single catastrophic event).

-darren

 

[darrenf]

[..] Maybe someone will come out with a manufactured back that is compatible with the touchstone?

The trick is the first part (which isn't detailed in this article) where I soldered leads to the charging pins on the micro-USB connector. Unlike the Pre, the Nexus S doesn't have exposed contacts in the battery area leading to those pins, so having a custom-made back would only solve the easy bit shown here and not the much more difficult step of soldering leads to the charging port.

-darren

 

[wirelessness#AC]

I had the Pre and really liked the inductive charging. I'm surprised it has trickled into more devices. Does HP or someone else have a patent on this technology...is that why it's not taking off?

 

[kalex]

Nice job

 

[hp79]

Is it possible that the induction charger will damage the NFC circuit in the long run?
Because the magnetic field, there will be some electric generated on that NFC antenna too. This is what I'm worried about. I might just try putting the back cover with NFC on my touchstone once I get it and measure the voltage of NFC contacts from the cover.

Was the palm touchstone coil too thick to keep it inside the back cover with the NFC together? Does it have bad effect this way? I want to mod my Nexus S, but I want everything inside the cover.

Thanks for your wonderful write-up.

 

[darrenf]

I worried about the same issue (touchstone inducing current in the NFC coil) as well as the possibility of the NFC antenna energizing the charging coil all the time and running down the battery.

I haven't seen any evidence of problems. I've been running this setup for three months without trouble. Battery performance is the same with NFC turned on or off. I would guess that NFC would be a little more sensitive without the charging coil in the way, but I didn't run any tests.

In the first iteration, when I had the charging coil inside the case, it fit well, but I had to be *sure* that the wires didn't cross or pass under any part of the coil or it bulged at that spot.

Because the back of the case is slightly rounded across the width to begin with, and because the back is slick, it didn't stick to the touchstone as well as I would have liked in the car. It worked great on my desk and was drop-dead sexy because the coil was concealed in the back, but in my car it's mounted almost vertically so sticktion was an issue if I hit a big bump.

While the externally-mounted coil is a little less attractive, it adds grip in the hand *and* sticks like glue to the touchstone.

Thanks for the props. Feel free to ask any questions.

-darren