Logitech K750 wireless solar keyboard mod

I've had a few Logitech K750s now and while they usually work great for about 2-3 months, they're notoriously bad for recharging and keeping a charge. Tired of dealing with this obvious flaw, I decided to "upgrade" mine to be a USB-charged battery-powered keyboard with a heftier battery.

Peeling off the glued front cover. (I salvaged my solar panels for another project since I wasn't going to need them)

Removing all the screws and popping the clips reveal the insides of the K750,
an Atmel ATXMEGA32D4-AU and a Nordic NRF24L01. 

On the reverse side of the PCB Logitech was nice enough to break out power & SPI to pads for us.

Without any disassembling, the original battery holder is removable with a little bit of prying and wiggling.

Taking some measurements with calipers and a little time in SketchUp, I recreated the battery insert with channels for some stripped solid core 22AWG wire to slide through. This provided a nice contact to the battery terminals inside without requiring any disassembly and a case for our new battery. Out with the old 65mAh coin-cell, in with the new 1000mAh LiPo (that should last a little longer and I had one laying around already...)

The 90 degree block was made as a separate piece due to too much friction while inserting the wire.

The first prototype, printed @ 230C/105C HBP. 0.2mm 50 travel/80 feed, 2 shells.

The original ML2032 is a 3v whereas our new LiPo is 3.7v. This shouldn't be a problem since the old solar cells output 4v to charge the coin-cell on the same circuit, so the regulator should be able to handle 4.2v. Testing a few points around the board, it appeared everything was powered by the regulated 2v "V_MAIN".

Plugged in and functional.

There's plenty of room for improvement but it serves it's purpose for now. It could use a bit of work in the aesthetics department and I originally planned on adding a LiPo charger with a microusb connector but it's been 3 months now and the battery voltage has only dropped ~0.05v, I'm probably not going to be needing it anytime soon... Maybe 1000mAh was a little overkill.


Like any wireless keyboard, "secure" or not, encryption on these devices are a joke. But I'm sure I'm not the only one who sits at their PC in a Faraday cage right?