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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOHdAia2su2SgW7UBrBP0rWPM87YZibuOICc5COdY22H_ItdWkuO_pcq_Zybn7ZpJc7xh-6NFKoz-NrZPqZMV3uLJtfqKtFAZb-T-KWS0c09r3MxR2fIJ_W09xNXDk7bFwSpogqoNb0qxP/s1600/keyboard_0000_keyboard.jpg) |
Peeling off the glued front cover. (I salvaged my solar panels for another project since I wasn't going to need them) |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJrUZeLaOL1FTrLG4RhULeEkC3oY1y9NchAIcsIWQIa8uSf6JM9vk7GsBBZv6wdBx29jBmabs2SAuHmdvitJ3xWJKji8pG7lSCrww88FH5plgUZtZDCdJsEKPBkqpVHkq8epfV19Pe2pUV/s1600/keyboard_0003_circuit_board.jpg) |
Removing all the screws and popping the clips reveal the insides of the K750,
an Atmel ATXMEGA32D4-AU and a Nordic NRF24L01. |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgWzE3Syg7GsBA4-zuu96pwAz0F-t9qwXIaplDjGcZ2LVts7uEDhGYOd_YZ8TW4Tfc9e-KU2GMQvJ9X6T-Cbjiw8Il3WYv64Jg1uIOIzQLsyG0BqZZxZFoufPNoEzOKiiJFYDnVFNGZAdHt/s1600/keyboard_0001_keyboard_closeup.jpg) |
On the reverse side of the PCB Logitech was nice enough to break out power & SPI to pads for us. |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEisetWbRtpgjMyKD0dehBsy0-vhBtIj7h8nU8N4gwcnmi91WTHGA2IK5tuD-nuFh2z29vHFIBuybCPTpU5fmCXlhG1Yc4SrDMVtwJzC0IpgTZdBfCVCJ8FG4S2vwFE3iyUZQ7BN_m-tcnfm/s1600/keyboard_0002_original_battery_holder.jpg) |
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...)
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiLJiNE2_ncYu5-EnRcPdOvJcI0Oj-nUNCzk3GLipqzscKV3zFXQ2SiEIQjFX35sX0InuN7dSBsTCrqAt830CeaFHqm-9XGex2YqzPW_oWfdjPCNZ41-YfQiAOi5VIBg3hgs1QNDQ8MDfD7/s1600/battery-holder.png) |
The 90 degree block was made as a separate piece due to too much friction while inserting the wire. |
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh6gj6cubU0efPLwIEXXwms044KNJ9oUp_H6DmB3o0GSyK2HilXc_0XiUPwe0eT10Jvnp-XwS9OC8wflsSCPksxSJNHVTZo47VBik0UW-oE87Qr128l_M62iGwPjnW4Z9HP_jpS9pgg-cl5/s1600/keyboard_0004_battery_replacement.jpg) |
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".
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgfFG-1J49GlmBIM5rCaI4VmX-7hkcEK5pWvhnyx2mrzIDL5p3Kow7n91SP6dNw4x0vf9B9WWD9HtriUW9VELAujbP2hV90cU_XLoyjhoMQro2vAabqr95dqZFDr0d6Xtow58s2zDixEFAA/s1600/keyboard_0005_inserted.jpg) |
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?
Could you elaborate a bit on the PCB test points? My K750 has gone dead and I would like to check, if there is some life left on the PCB...
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Björn
Nice! want to do the same, but with a 18650 on top. Can you share the stl for the insert clip? Thank you in advance!!
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