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Automatic charging and wireless hotspot in the car

A friend gifted me an Qi-based inductive cell phone charger that worked sporadically. I opened it up and found a couple bad solder joints to the power connector and cleaned them up. My cell phone, a Samsung Galaxy S4, supports wireless charging with an addon receiver coil. I picked one up off Amazon for cheap, slapped it in the phone and used the charger for about a week before I got irritated at how slow it charged compared to the stock 2A wall wart. The phone reported 640 mA during wireless charging and a whopping 1.8 amps when plugged into the wall.The wireless thing is clever, but positioning is critical, the range is pretty bad (<5 mm) and at less than half the output of a traditional plug it isn’t terribly useful.┬áThe one place it would be useful would be on the road.

Long ago I replaced the conventional radio in my car with a touch screen and several iterations of computers. I settled on a 7″ android tablet as it does everything I want much more efficiently than regular computer. I rely on wireless tethering from my phone when I am on the road as the tablet doesn’t have its own cellular internet access. The problem is that using the hotspot feature sucks up a lot of battery on the phone, which means I have to plug it in everytime I get in the car. I also have to turn the feature on every time I want to use it an off when I leave the car. In comes wireless charging. 0.6 amps is more than enough to charge my phone and run the hotspot, so I set about finding a good place to embed the charger. My car has a small cavity in front of the gear selector that I regularly drop my cell phone into while I drive. The spot nicely cradles the phone and keeps it from moving around with changes in acceleration. I pulled apart the dash and set about wiring everything up. Much to my disappointment, I was unable to just slap the transmitting coil underneath the plastic tray. The tray itself is made of thick plastic and the phone wouldn’t pick up the signal with a case on. To get around this, I used my CNC mill to shave out a pocket for the coil and *preseto* I was able to charge the phone with the case on.

I also embedded a permanent magnet to signal the phone that it is in the pocket. I programmed the very useful android app Tasker to turn on wireless tethering in the presence of a strong magnetic field and off again when the field is gone. You can actually use wireless charging as a trigger, but I found it pretty unreliable. The magnetic method works every time. So now all I have to do to charge and pair up my phone to my car is drop it into the little cavity. No cables, no fiddling with settings. I am pretty happy with this one! -Steve

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