Wednesday, December 29, 2010

Some CGM results and how to fix your reed switch in your wireless computer.

I've just completed my third week of using the Dexcom 7 continuous glucose meter. So far the values are mostly in the ballpark with my glucose meter. At least within 20%. Which is OK for the most part. And it is mostly continuous. As you can see below in the nighttime plot for this day sometimes when I apparently am turned one way it misses some readings.

The next two plots also confirm interesting trend that I was thinking may have been happening before I got the CGM. The first plot I went to bed with a reasonable level of 160ish. Then it jumped around a little bit and drops until morning. Most nights it stays pretty constant though. I ate breakfast and it rises and then drops until lunch. This is all good.

Now the second plot.... My sugar was a little high at bed so I took a small amount of Humalog along with the Lantus. This seems to happen even if I take the tiniest bit of Humalog at the same time I take Lantus. I have more plots that look just like this when I took Humalog.... The sugars drop and stay at a constant low level all night pretty much. Sometimes it takes a while to figure out my name when I finally wake up. And then when I eat (even a light breakfast) the glucose shoots through the roof even with extra insulin after breakfast. This plot is actually higher than my other meter read on this day but the point is still clear. Notice that it also takes a long time to drop back down.

So after looking several different nights data..... The key is to not take any Humalog with Lantus at night and try to go to bed with a level of around 150. And hope I can wake up in the morning.


OK so I got a Garmin Edge GPS cyclocomputer. It basically does everything. Tells you heartrate, cadences, speed, position, altitude, maps your ride etc etc. The weak link? OK other than leaving it in your pocket and washing it.... It worked for a month or two then died. I sent it in and they gave me a new one for a fee. They didn't tell me what killed it but probably the washing. So anyway the real flaw is shown below in the speed/cadence sensor. The speed sensor picks up a signal from a magnet attached to the spokes to give better speed data if you are in thick trees or whatever and don't have a good GPS signal.

On the little arm that pokes out and can be adjusted to get it close to the magnet on the spokes is a reed switch attached to a small circuit board. This is a cool little thing in itself and is shown below. As the magnet passes it it creates a connection and completes a circuit which then sends a signal to the computer to compute speed. A similar circuit collects cadence data.
So what happened to mine and what is the flaw. The reed switch is a glass tube filled with inert gas. So it makes sense that the last time this thing worked was just before I bumped the sensor and it got whacked by the magnet. When I took it out the glass was shattered.

So the key to opening up your GCS 10 it to take a chisel or something and carefully split it open on the seam. I left it attached to the cadence part to help stabilize it. Then I just pulled the plastic back to reveal the switch attached to small circuit board and pulled it out.

The next step is to find an old computer that broke and can be cannibalized. I had just the thing--pays to be a pack rat--on the broken one below. So I soldered the board below to my unit and slipped it in.

And here I am slipping it in. One thing I noticed was that this reed switch had a bunch of glue all over it to not only keep it stuck but I think to keep the glass from breaking as easily as it was completely encased in the plastic.

I hooked it all up. Tested it out by passing a magnet past it and the little test light on the sensor flashed green. Good to go! So I slipped it in pushed the cover over stopped working. Damn!
I thought the little circuit boards were close enough in size but this one was just a hair longer and it broke when I pushed it in. Damn! Luckily I had another old sensor that took apart. I soldered that reed switch on to the original board and tested it out. This one had some rubber between it and it's circuit board to keep it from breaking as easily.... If only Garmin would have put something in there to keep it from breaking so easily. If you look around on line or forums there a lot of these things breaking! So I put this one in and it fit nicely and all the electronics tested out. So now get out the epoxy and.... Damn epoxy has gone bad..

Time to get out the other epoxy and mix it up on the seed catalog....

Now spread the epoxy on the arm and press back together and clamp to cure. And let's see how long this fix works. By the way a new GCS10 goes for $37 with free shipping on You can also buy a reed switch here for $1.50 + SH if you didn't keep parts around.

So the moral to this story. Don't get your sensor too close to the spoke magnet and save all those old cycle computer parts when they break. And thank your spouse profusely for letting you solder and epoxy on the kitchen table.


L. Robertson said...

I'm not very computer literate and know I posted you a question about type 1 diabetes (my 10yo daughter) and I have no idea where I posted it and no way of knowing where to read if you respond. So, if you have advice and have time I will look to this spot since I now know where I'm posting.... Sorry...

Jarral Ryter said...

I was just in mexico so just checked my messages so hopefully I can help..... I have done a couple of things to try and keep the electronics warm enough to work. I toyed with something like a bra to hold it close on a backcountry ski race as I had my avalanche beacon and all at my waist. I ended up using a bag that i made to hang under my inner jacket. I also layer clothes a lot. So on an inner jacket with a pocket I put the device and then that is covered by another jacket and shell. I don't have much of a problem with this unless the temp drops below zero. I think if she had a jog bra that fit and you slip it in that or a inner coolmax shirt with a pocket on it she should be fine. If it is that cold she would probably be in the hot tub anyway! Good luck and just keep it all near the body and she should be fine. The good thing is that if something really weird happened and the meter froze she wouldn't go low. you can email me directly at and/or you could swing by our house for some ideas....