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I am trying not only to power wirelessply a dermatology device "SkImager", but also to make open-source battery-powered, wirelessly-charged skin dermatology device including a Rapberry Pi, a HD webcam, multispectral LED ring and data communication over WiFi connection to a webpage.
One thing is something that is made in a research project with dedicated electronics and unlimited resources. But for electronics hobby people a device should be made using popular and readily available components/modules that most people can get running without investing too much effort. Some of such brilliant things that recently have changed the hobby electronics world are Arduino and Raspberry Pi modules. Nowadays it is often cheaper and faster to buy complete electronics boards online than to order individual parts and solder.
One of suitable controllers for the dermatology device would be ARM-based single board computer Raspberry Pi (RPi) that is popular and reasonably priced. Within Wireless Power Challenge I used the bonus voucher to order a RPi from Element 14. Raspberry Pi can be powered from USB drawing less than 0.5 A. I tried to power RPi from wireless power kit RX board just by taking 5V output to the RPi USB connector. Raspbian Linux was booting from the SD card and LED’s were blinking like usually. Ethernet connection was working and I could log into it using SSH and also could browse the webpage generated by apache2 server.
When I tried to attach a USB Logilink WL0085 WiFi adapter and a Logitech C270 webcam, the Qi 5V power was not stable enough and RPi was not booting.
So, as expected, an intermediate step is needed, namely a LiPo batter pack. During discharge LiPo voltage drops from 4.2 V to 3 V and can not be directly used to power RPi. Voltage has to be boosted to 5V. I am grateful to Farnell that could order a very nice 5V step-up buck regulator LMZ10503TZE-ADJ/NOPB.
For charging the LiPo I planned to use a LiPo charger chip BQ24075TRGTT that converts 5V input voltage to 4.2 V maximally allowed voltage of a LiPo that is a very critical for fire safety as lithium rechargeable batteries may self-ignite if the cell voltage becomes higher than 4.2V. Charging current of this chip can be set 0.5A.
Using the both above mentioned chips would require a day to design, etch and solder a PCB board. Meanwhile an easier solution came to mind. Popular gadget on Ebay is solar-charged backup battery pack. This gadget already includes the LiPo cell, LiPo charger and a 5V step-up regulator. It can be used during mountain hikes, but charging from solar panel is usually very time consuming and it can be charged much faster via USB.
During last years I have ordered several such backup battery packs. Some of them were cryminal as they charged LiPo from USB 5V via a resistor without any controller chip; and step-up regulator did not not shut down at the LiPo lowest recomended voltage of 3V quickly destroying the LiPo. The LiPo pack inside is often smaller capacity than specified on the box. But some emergency power packs are really nicely and correctly made.
One thing that I don't understand: these battery packs have no OFF switch and generate 5V all the time meaning that there are some small losses all the time but they are quite insignificant.
Raspbery Pi with a WiFi stick and a webcam runs nicely from backup power pack. WiFi stick consumes 1 W and Webcam consumes 1W.
Next I have combined the backup battery pack with the Wireless Power Kit. See the photo below. It is a proof of principle setup. From a bash script I can program to take a photo from webcam using fswebcam program and can control GPIO pins that will be later used to switch on/off multicolor LEDs.
I see that the electronics draw more current than the Qi charger can supply and voltage over LiPo decreases with time. So I will not be able to make a medical device that can run 24/7 using this configuration. One possibility with less current consumption is to use a 5x5 cm pocket router WR703N running OpenWRT Linux. It has USB connector where USB HUB and webcam can be attached as well as Arduino-nano controlling the LEDs. I have been blogging a lot previously on the use of this router for home automation:
http://www.instructables.com/id/How-to-set-up-OpenWRT-on-a-pocket-router-WR703N/
Multispectral LED ring
Skin chromphores: melanin, bilirubin, hemoglobin absorb different spectral regions of light. Multispectral LED ring module including IR, RGB and UV LEDs could be a snap-on tool for microscopes and cameras not only for medical purposes, but also, for example, to examine minerals, vegetation, fruits, old books, money and paintings. UV light causes fluorescence of materials, while many things, like hidden text on paper, become visible only in the IR light. Crossed polarisers will be used to efficiently eliminate backreflected light and improve the image contrast. Such multispectral LED ring could be a possible product on Kickstarter and Ebay.
Attached file contains Eagle board. It took a while to learn that Eagle can draw in polar coordinates. One can enter commands from keyboard.
move R1 (P 33, 15)
This moves resitor at distance 33 mm form origin and places at angle of 15 degrees.
rotate r-15
This command rotates part that is selected counterclockwise by 15 degrees.
For visible I used SMD Super Bright LEDs from Farnell and 375 nm UV LEDs from Conrad.de.
Resistor in series with each LED is adjusted that every LED draws 20 mA from 4.5V. This makes total 80 mA from one colur LEDs wired in togather on the back side of the PCB.
AtMega328 is specified to deliver 60 mA per pin. Series resistors differ for different colors as each color LEd has different forward voltage.:
RED 120 Ohm
GREEN 100 Ohm
BLUE 68 Ohm
WHITE 68 Ohm
IR 940 nm 150 Ohm
UV 375 nm 62 Ohm
Arduino nano switches on LED colors one after another.
Looking at 50 EUR note one sees thin hairs flurescencing green and blue at UV excitation.
Commercial Qi transmitter supplies power. 0.5A flows when all LEDs are on.
This LED board will be used with Raspberry Pi after making power transistor switch board. Next step would be also to add crossed linear sheet polarisers and webcam.
Webcam infrared conversion
If I look with a webcam at 940 nm IR LED it is quite dim. Silicon photodetectors should work well up to 1.1 um.
Webcams usually have an IR filter after the lens. Without such filter things would look too red in sunlight. For some IR cameras with IR LED illumination the filter is not so strong.
Filter looks like a thin blueinsh glass plate. This glass plate can be remived (breaked). I use Logitech C270 or 9xx series webcam.
The image might look less sharp in all colors simultaneously because of chromatic aberation as the webcam lens was not designed to operate both in visible and IR. Best results are obtained with a lens having marking "IR" on it. Veins show up better in IR as longer wavelength penetrates deeper inside the tissue.
CNC machined parts
Last weeks I was working on putting togather a hobby CNC mill by Zen Toolworks. It allows to use 3 mm diameter mills compatible with Dremel tool. I spent some days troubleshooting why the CNC mill was loosing steps until found in google that problem was in optocouplers. It is fun to make parts that would take ages to be made by hands. Later I upgraded the mill with a 1W blue laser. Aluminum holder and heatsink for the laser module were also machined on this mill.
Using the laser I cut rings from the polariser sheet and plastic holders for crossed polarizers and the webcam.
I am continuing to work on this open-source dermatology device and update this blog.
Next blog entry: 10 Results and Summary on Wireless Power Challenge
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