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RX coil miniaturisation to fit the "SkImager" dermatology device
I used dremel to cut down the extra ferrite material around the RX coil. I used WE coil nr 760308204 that is the smallest. I left ferrite edge ca 1mm larger than the copper coil and with reduced size it fits under the dermatology device handle bottom, where the traditional charging contacts are placed.
The RX board delivers the same power if magnetic attractor is removed from the RX coil. It works also if a knife blade is placed on the top of the RX coil with removed ferrite shield, but the knife blade becomes warm. Removing the ferrite shield should influence the inductivity, but it looks like that issue is not a problem.
Receiver board miniaturisation
Thanks to Eduardo Iscar Rüland who designed and made available receiver board drawing. I download it and opened with Eagle.The old Eagle version 4.1 could not open the board and I had to download and install the newest version that is possible to use as a freeware for small size two-layer circuit boards. As the Challenge ends next week there is no time to order receiver PCB, but I could probably etch PCB myself. Another solution would be to use thin enamel wire and try to solder the chip on a breakout board, but it is a juvelier's effort. Just found out that company Adafruit offers Universal Qi Wireless Receiver Module for $14.95, but there is no time to order it.
Finally, I decided to try to cut down the EVK RX board to fit the dermatology device "SkImager". Ideally the RX board should become equal or smaller than the RX coil diameter.
First I removed the testpoints and jumpers. I had to move some parts closer to the chip to decrease the PCB size, for example, R11, C1-C5, R4.
There are many tiny dots on the circuit board. They are vias so small in diameter that no light goes through. Keep in mind the vias if cutting the board.
After cutting the board the ground plane had to be united in several places.
I used 0.2 mm diameter enameled transformer wire for connections. When it is heated by soldering gun at 450C, the enamel locally burns away and wire can be nicely tinned and soldered.
The TI PCB is very high quality with smooth copper trace edges and nice paint protection. Really a pleasure to work with.
It took me 5 hours to strip-down the EVK receiver board as I tested if it is still working after each change.
Finally the RX board became smaller than the RX coil and was still able to supply 5V at 1A.
This was nice training to use SMD. Now my hands are much less shaky. I could borrow nice tools from my colleague: a thin tip solder wire and soldering iron and hot air reflow station to remove SMD parts and magnifying glass. I guess with this practice I will be able to design my RX circuit board soon. I have erased parts from the original evaluation kit schematic and the circuit looks quite simple.
Installation on the SkImager handle
Skimager needs up to 0.9 A when it is running and charging the LiPo, and 0.5A when it is just charging. Here is a photo from a test that wireless charging supplies enough current.
I unsoldered gold-plated spring battery contacts from an old mobile phone and mounted it on a piece of a prototyping board cut to match the Skimager handle. It was possible to add wireless charging without opening the Skimager. The RX board and coil I fixed using double-sided tape. When everything will work fine they will be epoxied into one package. RX assembly is taped to the SkImager handle for present testing.
Wireless charging works very nicely. Conventional charging requires to align the SkImager to mach the charging contacts. On Wireless charging board Skimager can be placed in any orientation which makes it's use faster.
As the wireless transmitter I used evaluation kit TX board placed into a plastic DVD holder. SkImager is held vertically centered above the TX coil using a piece of plexiglas pipe.
For more professional look holder a stand including the wireless transmitter board is being designed for 3D-printing.
Representation photos showing result of this Wireless Power Challenge project
Wirelessly charged skin dermatology device "Skimager" developed at the University of Latvia.
We are designing a holder stand for SkImager with more professional look including the wireless transmitter board.
A new version of SkImager enclosure has been recently 3D-printer and will be assembled with Qi wireless charging coil and board mounted inside it's.handle.
In our institute an extremely talented collegue Janis Zaharans has put together a 3D printer controlled by a Raspberry Pi and Arduino Mega. 3D printer allows one to create amazing shapes that would not be possible to make by hand. No dust, relatively quite, and can operate besides your office desk.
If you have time please visit the printer website and see what it is currently doing:
http://85.254.232.77
On June 25 we have 3D printed plastic adapter for the RX board and coil. It fits nicely on the SkImager handle. Printing uncertainty is 0.2 mm.
Transmitter board miniaturisation
We will design in Solid Works and print out the stand with included Qi transmitter. I have cut down the TX board under the TX coil, and removed aluminum plate using hot air gun. That reduced TX assembly thickness by 3.5 mm. Now the thickness of TX electronics is 6 mm.
Home-made receiver board
My collegue Janis Zaharans helped a lot on progress. We have designed a tiny Eagle wireless receiver board measuring 10x15 mm. Miniature RX board Eagle .sch and .brd files are attached. "Ratsnest" is not shown in the picture. Noticed two corrections 2.4k resistor can be jumpered to gnd and instead of 66k should be 10k.
PCB was made at home using hot-toner transfer method, etched using FeCl3 and soldered using hot air reworking station. During soldering with hot air the RX chip and other components pulled themselves into place by surface tention. The assembled RX board works! At 0.9A the RX chip gets hot but it is possible to keep a finger on it. Heat is transfered to the copper on backside through via holes.
Finally everything is assembled into a 3D printed adapter fitting on the "SkImager" handle and tested that it works nicely.
Next blog entry: 08 Short introduction to dermatology and dermatoscopy using multispectral imaging
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