Hand Hygiene

At the time of my departure for Massachussetts, we have yet to create a working prototype badge for the medical professionals to wear. We were trying to implement several important features that previous mote badges lacked:

• we wanted the contraption to fit entirely inside the old pager casing, eliminating the USB port and the charging socket;
• we wanted the ipod battery to charge from a USB with an internal MAX1555 circuit;
• we wanted to be able to see the LEDs from the mote and the LED that indicates charging
• and we wanted the mote to be capable of disconnecting from the cheaper housing, however we allowed the mote’s connector to also be attached to the battery, so long as the battery only powered the mote when plugged in.

We created several prototypes to attempt this, all of which were designed around a printed circuit which had convenient holes to allow the three placement pegs in the bottom of the pager casing to make a tight fit. Unfortunately the holes were pressfit and the way our spacers worked out it is dangerous to press hard enough to get them in; larger holes are needed in the future, but are not yet implemented in the eagle file. We got this idea from the original pager guts which are still lying around the lab.  These guts are usefull, I’ve included pictures so they will be easy to identify. I left the guts in the top left drawer at the microscope soldering station.

The basic idea of our solution was to mount the mote upside down over the badge board with the lithium ion rechargeable iPod battery sandwiched in between. The original pager guts have convenient short header connecting them, but we decided to connect the mote and board, after some fiddling around with 30 gauge wire and some tiny hirose connectors, using 1mm spacing ribbon cable. Our first attmept to put this stuff in got every pair of two backwards, but this version you are seeing is on the most recent prototype. The wires are directed as such:

1. Mote Ground
2. Mote Power
3. unconnected
4. Analog input 0
5. USB right data pin
6. USB left data pin
7. Battery Ground
8. Battery Power – this wire is red, the others are grey

The mote is held onto the badge board by three 2-56 screws inside .312 inch aluminum spacers. The battery is between the two, and the usb mini port is on a tiny daughter of the badge board with the charging light. this board is all single sided.

I’ve included the eagle cad files for this board as eagle documents relabeled as jpeg pictures to bypass the blog security features. They will be usable if the .jpg suffix is removed. They are also on the lab server.

Fitting the badge in the case

The badge board has, as I said three holes that are pressfits to three plastic pegs that stick up out of the bottom (larger piece) of the UIHC pager body. By having these holes press fit the board is very hard to get in, especially when it is screwed to the mote and it becomes impossible to directly push on the pressfit holes. Because of this I think later versions should widen the holes. Also, the ribbon cable connector is further back in the board file than in the current boards because the ribbon cable pushed against a wall and required the removal of plastic to fit. We have tried to avoid changing the plastic badge cases because it will be a manufacturing bottleneck when we make a bunch of these things. We do unfortunately need to remove a lot of plastic from the top, but we were trying to leave the bottom alone because it is harder to work on than the top. There are several working tops lying around, and the creation proccess followed the very simple plan of cutting off whatever piece made the mote not fit, so this should not be too hard to replicate. Do note that the fit in the end is still not very good. We may want to find shorter spacers for later versions, because it is the height of the apparatus that is making it hard. If we do shrink the spacers it is important that we find a way to isolate the wires connecting to the usb data lines from the top of the mini usb because the aluminum mini usb housing could short the two and make the mote unprogrammable. This would not be fatal, but it would be a pain to open all the motes before programming.

At the time of writing we haven’t figgured out the size of the batteries yet, but we were using iPod batteries for the smaller iPods. The batteries were much thinner than we needed, but getting a width and height was hard. I shorted the last working battery in the lab when I was trying to solder directly onto its tabs, so we will need to get more from Ted.

What we need to make a working prototype

We are very close to a working prototype, one board, picutured above, has everything we need, except one battery (which I shorted) and one working ribbon cable connecter (which I used to the point of supergluing). All one would need to do to replace the components that we know have failed is to desolder the right angle header which is superglued to the ribbon cable plug, replace it with a new one,  cut off the ribbon cable, put it in a new ribbon cable plug in the same orientation and press it using the DFM lab’s little green arbor press, then solder on a new battery-positive to ribbon-8 negative to ribbon-7.  Some filing of screws would probably be neccisary to get the thing to fit properly, and even then the ribbon cable plug will be right up against the wall, because the prototyped board used the wrong size estimate.

Constructing the prototype after that one

After the first prototype is finished it is important to understand our use of the reflow skillet. Our skillet is basically a resistor with an on off swich connected to wall power.  While it may appear as though it has a full range of temperatures from 0 to 500 with an extra high setting, I have taken the electronics appart and the switch is very much binary.  So it has to be carefully monitored with generous solder samples in multiple places . The overall procedure is to borrow the solder paste from the electronics shop, because our solder paste is very sand like, and will not stick to copper traces; to use a toothpick to carefully apply this solder paste to the pads, while using the tv-camera microscope to see what you are doing more easily; to generously add samples of solderpaste in unused areas of the board to determine the surface temperature of the board relitive to the reflow temperature of the paste; to  place your components onto the solder paste globs using tweezers–note that the solder paste will pull the components towards the proper placement once it melts;  to delicately  place this board with components on the skillet; to put the lid on the skillet; to turn on the skillet; to wait until all the samples flow; to delicately remove the board; and finally to declare victory. It was my experience that going over the board with an exacto knife to remove potential short is a good use of time. The daughter board is connected to the main badge board by .1″ pin header. To attach it, slide the black plastic orientation bar to the bottom of the header; push this up from the bottom side of the mother board; solder it to the top of the mother; put on the daughter board; solder the daughter from the top; remove the orientation bar and solder the bottom. This path avoids ever misaligning the header, which would be easy to do if one removed the black spacer too early.

Changing the board file

The board file was made in eagle, a program by CADsoft. The lab uses the freeware version because we have decided that hand sanitizer research is not a commercial operation. Eagle works by using vast libraries of electronics parts to define boards. Several of our parts use special libraries-created from the data sheet of the electronic component we needed. We tried to avoid making new libraries for parts we already had in another library, but I did add a small square pad orientation to the existing library containing .1 inch header. The parts are added onto the schematic first, and then they are automatically added to the board through the switch to board command. The trick to making sense of eagle is to make all the circuits in the schematic first, because eagle refuses to create shematics to match boards and having a matching schematic is so useful for debugging.  Also, there is a library for the ground and power symbols–definately worth using in the schematic.

Ted’s long term plan for the motes

Ted, the lead programmer for this project, has expressed interest in shrinking the pager boards even further than we are curently doing by ordering motes with fewer features and even smaller components. It may become the lab’s job to build a new housing for these badges. It might also be possible to build the current cirucit into the mote–and even add new leds or other such features.